- He is standing at the gate.
- He is standing among the crowded place.
If both are wrong what is the right usage?
Various dictionaries, for example Cambridge, define among along the lines of
in the middle of or surrounded by
This definition would lead one to believe that being in the midst of a crowd would endorse the sentence
He was standing among the crowd.
However, virtually all of the examples in those dictionaries use a plural noun in describing what he is among. The term crowd is a collective noun but connotes the whole rather than the individual members. Standing among the crowd just sounds wrong.
The phrase the OP offers
the crowded place
is clearly a singular concept - a place that happens to be crowded. As such, it would not be an acceptable usage.
First sentence is correct. Second sentence should be "he is standing in the crowded place." Although even then it sounds like a sentence constructed by a foreigner. More common or natural would be "he is standing amongst the crowd"