Top Gun was released in the age when the PG-13 rating was brand new and no one really knew what would go in that space. For movies that were released in the early 80s or before, you need to look at ratings differently.
G = G/ Modern PG
PG = Modern PG-13
R = R
You look back at a lot of movies from then and they'd slip a bit with their ratings. Short Circuit would probably be PG-13 today (not that I'd get made with that script, but that's a discussion for another day), The Land Before Time would probably be PG just because of some thematic elements, and we all know Raiders of the Lost Ark and Indiana Jones And The Temple of Doom would be PG-13 since Temple of Doom is the movie that helped birth the PG-13 in the first place.