GameYob doesn't run GBA games, for GB(C) games though, yes you can. I think the default fast forward key is L, but you can change it if you want from the menu (tap the screen by default) -> Settings -> Key Config. If you want speedup though you probably want to use the DSi version (included in TWiLight Menu++ or here (or Universal-Updater)) as it runs faster than the 3DS version for whatever reason iirc.
For GBA games if you're on n3DS you might be able to get some speedup using mGBA or gpSP in RetroArch, those already have a hard enough time running at 100% speed though that I'm not sure how much they can speed up.
Edit: Did a quick test, GameYob DSi gets about 400 FPS on the title screen of Pokémon Crystal, GameYob 3DS gets only about 200 FPS.
Forget about fastfowarding GBA games, the only software that could do that runs slowly even when it should run at normal speed, so unless choppy frameskipping is your idea of increasing speed, then no, it can't be done. Also this is a GB/GBC emulator. Please read the threads carefully before asking stuff