I'm planning on Speedrunning my old Gameboy Advance games, what are the rulings on GBA submissions? Are emulators allowed? Some of the games I used to endlessly play don't have listings should I go about trying to ad them or should I just run GBA games that already have people running them?

"what are the rulings on GBA submissions? Are emulators allowed?" Depends on the game's leaderboard rules and/or forum. If it isn't written down in the rules, ask the moderators.

"Some of the games I used to endlessly play don't have listings should I go about trying to ad them or should I just run GBA games that already have people running them?" Depends if you already have plans for rules and if you feel ready to moderate a leaderboard. For starters I can recommend to start with a game that's on the page and see how the community handles things. You can still add games later on. If you want to create a leaderboard, have a look into the knowledge base -> https://www.speedrun.com/knowledgebase/requesting-a-game