How realistic is the plane scene in Captain America: The First Avenger?
Though 'getting on/off aircraft through landing gear' is a favorite trope in movies, it is practically impossible in most of the modern airliners. Airliners are pressurized, and as a result, the cabin (and cockpit) area is in reality one big cylinder, cut off from everything else. So, no accessibility from the landing gear area.

For example, the following image shows the landing gear bay (the area into which the landing gear retracts into) of a Boeing 787. 



Image from dailymail.co.uk

As a result, though the list of stowaways in landing gear area is long, to the best of my knowledge, no one has entered the aircraft through it.