ArcEmu
Emulator with online support for iOS and Android, not cross play though.
guide not written yet
also on GBC
Nintendo Switch Online
Official Online method for Nintendo Switch Retro Games
guide not written yet
also on SNES · NES · GAMECUBE · N64
Parsec
A streaming platform that allows online multiplayer gaming by sharing a computer's screen.
guide not written yet
also on PSX · NSW · SNES · PS3 · Wii · GENESIS · PS2 · GBC · PS1 · ARCADE · NES · GAMECUBE · XBOX · NDS · SATURN · 3DS · N64 · STEAM · Dreamcast · PC+
Pizza Boy A Pro
GBA Emulator on Android
guide not written yet
RetroArch Netplay
Netplay feature of RetroArch that allows multiplayer gaming on various retro emulated systems.
guide not written yet
also on PSX · SNES · GENESIS · GBC · ARCADE · NES · SATURN · NGCD · PCE
Play it with people
Find people to play Game Boy Advance with
Be the first here to play GBA online. Open a lobby and the site pings everyone who plays it.
Nothing open right now. Open a lobby and the members who play it hear about it.