On locations it says that if I have a certain gba game in the gba slot a certain pokemon might come up. My question is, do I need the national for that to work? (sorry if its been asked already, I couldnt find it if its already asked)
Yep you would take this page for instance route 228 there is a link at the top linking you to the gba insertian section or you can just scroll down.
there is only 1 pokemon on this route that you can see with gba insertion (sandlash)
and you have an 8% chance of finding it when leaf green is iserted.
that is the reason you need the national dex because sandlash and the other pokemon you find with it are in the national dex.