bgb is a GAMEBOY / GAMEBOY COLOR emulator (why's this in GBA?)...
anyhow, you use the tcp/ip link in bgb:
run bgb
one of you must LISTEN (right click the screen, select Link -> Listen) and set up the IP/port (type in your IP and a colon and 8765 for example 12.3.4.5:8765)
the other person must then right click and choose Link -> Connect and type in the opponents IP address.
I know this works on the same machine, i've never actually tried over the internet before.