In my opinion there are cases where it is and isn't redundant. According to the Wikipedia disambiguation page a garage may refer to:

"Automobile repair shop, where vehicles are serviced and repaired."

It would be very strange to say that your car is parked inside the repair shop. Especially if it's on a car hoist with its wheels removed. Therefore I claim it is not a parking garage at all.

"Bus garage, a building or complex used for storage of buses when not in use."

The buses at this garage, or maybe (depot more commonly) is for the parking of buses. If you use the word "garage" as the place where the buses are kept when not operating, then parking garage in this sense is redundant because you are parking them in this garage(place where buses are parked).

"Garage (residential), a building or part of a building for storing one or more vehicles."

If you keep your car in your residential garage, it's a garage where you park your car, so parking garage would be redundant. However some people have a garage in which they don't park their cars, they might keep only tools or equipment, then it's not a parking garage, it's just a part of the house or outbuilding that may been designed to hold cars and repurposed for something else.

"Multi-storey car park, a building serving as a public parking facility."

If a "garage" means the area where cars are parked, then parking garage here is redundant.

"Filling station, where vehicles take on fuel or recharge"

This is a tricky one. I don't call a filling/service/gas station a garage at all. Here you would be parking your car. If a filling station is a garage and you park your car, even temporarily while you get fuel or buy a donut. Then maybe it could be redundant, but I'm not sure about this.

Is there an instance in which the use of the word "garage" as opposed to "parking garage" would imply something other than a structure where vehicles are parked?

Yes, see my explanation of a repair shop and residential garage.

Also, just because a term is redundant (can also be called tautologous), doesn't necessarily make it wrong or frowned upon. I'll try to find examples of such terms.

Wikipedia meanings of garage

In my opinion there are cases where it is and isn't redundant. According to the Wikipedia disambiguation page a garage may refer to:

"Automobile repair shop, where vehicles are serviced and repaired."

It would be very strange to say that your car is parked inside the repair shop. Especially if it's on a car hoist with its wheels removed. Therefore I claim it is not a parking garage at all.

"Bus garage, a building or complex used for storage of buses when not in use."

The buses at this garage, or maybe (depot more commonly) is for the parking of buses. If you use the word "garage" as the place where the buses are kept when not operating, then parking garage in this sense is redundant because you are parking them in this garage(place where buses are parked).

"Garage (residential), a building or part of a building for storing one or more vehicles."

If you keep your car in your residential garage, it's a garage where you park your car, so parking garage would be redundant. However some people have a garage in which they don't park their cars, they might keep only tools or equipment, then it's not a parking garage, it's just a part of the house or outbuilding that may been designed to hold cars and repurposed for something else.

"Multi-storey car park, a building serving as a public parking facility."

If a "garage" means the area where cars are parked, then parking garage here is redundant.

"Filling station, where vehicles take on fuel or recharge"

This is a tricky one. I don't call a filling/service/gas station a garage at all. Here you would be parking your car. If a filling station is a garage and you park your car, even temporarily while you get fuel or buy a donut. Then maybe it could be redundant, but I'm not sure about this.

Is there an instance in which the use of the word "garage" as opposed to "parking garage" would imply something other than a structure where vehicles are parked?

Yes, see my explanation of a repair shop and residential garage.

Also, just because a term is redundant (can also be called tautologous), doesn't necessarily make it wrong or frowned upon.

"They filed through the doorway one by one."

Any ordinary doorway would most likely only accommodate a single person at a time, so "one by one" seems tautologous. However as a matter of style it's fine, at least I argue.

Wikipedia meanings of garage

In my opinion there are cases where it is and isn't redundant. According to the Wikipedia disambiguation page a garage may refer to:

"Automobile repair shop, where vehicles are serviced and repaired."

It would be very strange to say that your car is parked inside the repair shop. Especially if it's on a car hoist with its wheels removed. Therefore I claim it is not a parking garage at all.

"Bus garage, a building or complex used for storage of buses when not in use."

The buses at this garage, or maybe (depot more commonly) is for the parking of buses. If you use the word "garage" as the place where the buses are kept when not operating, then parking garage in this sense is redundant because you are parking them in this garage(place where buses are parked).

"Garage (residential), a building or part of a building for storing one or more vehicles."

If you keep your car in your residential garage, it's a garage where you park your car, so parking garage would be redundant. However some people have a garage in which they don't park their cars, they might keep only tools or equipment, then it's not a parking garage, it's just a part of the house or outbuilding that may been designed to hold cars and repurposed for something else.

"Multi-storey car park, a building serving as a public parking facility."

If a "garage" means the area where cars are parked, then parking garage here is redundant.

"Filling station, where vehicles take on fuel or recharge"

This is a tricky one. I don't call a filling/service/gas station a garage at all. Here you would be parking your car. If a filling station is a garage and you park your car, even temporarily while you get fuel or by a donut. Then maybe it could be redundant, but I'm not sure about this.

Is there an instance in which the use of the word "garage" as opposed to "parking garage" would imply something other than a structure where vehicles are parked?

Yes, see my explanation of a repair shop and residential garage.

Also, just because a term is redundant (can also be called tautologous), doesn't necessarily make it wrong or frowned upon. I'll try to find examples of such terms.

Wikipedia meanings of garage

In my opinion there are cases where it is and isn't redundant. According to the Wikipedia disambiguation page a garage may refer to:

"Automobile repair shop, where vehicles are serviced and repaired."

It would be very strange to say that your car is parked inside the repair shop. Especially if it's on a car hoist with its wheels removed. Therefore I claim it is not a parking garage at all.

"Bus garage, a building or complex used for storage of buses when not in use."

The buses at this garage, or maybe (depot more commonly) is for the parking of buses. If you use the word "garage" as the place where the buses are kept when not operating, then parking garage in this sense is redundant because you are parking them in this garage(place where buses are parked).

"Garage (residential), a building or part of a building for storing one or more vehicles."

If you keep your car in your residential garage, it's a garage where you park your car, so parking garage would be redundant. However some people have a garage in which they don't park their cars, they might keep only tools or equipment, then it's not a parking garage, it's just a part of the house or outbuilding that may been designed to hold cars and repurposed for something else.

"Multi-storey car park, a building serving as a public parking facility."

If a "garage" means the area where cars are parked, then parking garage here is redundant.

"Filling station, where vehicles take on fuel or recharge"

This is a tricky one. I don't call a filling/service/gas station a garage at all. Here you would be parking your car. If a filling station is a garage and you park your car, even temporarily while you get fuel or buy a donut. Then maybe it could be redundant, but I'm not sure about this.

Is there an instance in which the use of the word "garage" as opposed to "parking garage" would imply something other than a structure where vehicles are parked?

Yes, see my explanation of a repair shop and residential garage.

Also, just because a term is redundant (can also be called tautologous), doesn't necessarily make it wrong or frowned upon. I'll try to find examples of such terms.

Wikipedia meanings of garage