Present participle has a form of "be" verb before it.
I am eating (past simple: I was eating)
He / she is eating: (past simple:He / she was eating)
The present participle expresses an action that´s going on at a given period of time.
It could be:
Present participle: I am eating. (Happening now)
Future continuous: I will be eating at 7:20pm. (This will be happening at a given period of time in the future)
Past continuous: I was eating at 7:30am (This was happening at 7:30am)
Gerund has no "be" verb before it, unless it´s a question though.
Gerund can not use in the past, you´ll need an auxiliary verb before it to do so. This is because gerunds are nouns.
Some nouns derive their names from verbs, such are considered gerunds.
I love something. ("Something" in this sentence is a NOUN")
Now, let´s substitute the word "Something" for any other noun:
I love apple
I love football (Football is an event, a sport an activity, a sport, a noun.)
I love people
I love eating (This does not mean that I´m eating now)
I love fishing
Similarly,I was thinking about eating the apple falls in the same category
You think about "SOMETHING". "Something" is a "NOUN". Substitute the word "Something" for any other noun to make a complete sentence.
I was thinking about eating.
I was thinking about dancing.
I was thinking about life.
I was thinking about my family
I was thinking about eating ... ("the apple") is just a complement in this case.
I love dancing (This does not mean that I even know how to dance, perhaps I just like watching it.)
I hope this does help.