I think it is only the Centerfield gate the opens early. I know when I was in Cleveland we got to the game early and they had one gate open and the concourses closed until an hour before the game. Also Kings Dominion opens their gates at 9:00 but doesn't start rides until 10:00. Nissan Pavilion has also done stuff like this. So really it is nothing special to the Nationals or even baseball. It is just a thing places that entertain large groups of people do.
Right. Seating areas open up 90 minutes before the game, period. That's the MLB standard. Somehow they are also allowed to open up the outfield plaza area an extra hour earlier.