You can play Borderlands co op on Garena but for that you will first have to double check your port forwarding and firewall settings.You can follow our instructions on how you Borderlands Portforwarding and Firewall Guide.
After you have correctly configured borderlands firewall settings and have forwarded all the ports, you can follow the instructions below to play borderlands on Garena.
Setup Borderlands for Garena
Update to the latest version of the game. *Or the one on which everyone at Garena is playing* in case.
Open Garena, Login, and Join a room. Now click on settings, choose Borderlands as game and set the correct executable file i-e borderlands.exe.
Now run borderlands and choose Lan Game to join any hosted server. Or you can always host a server of your own for others to join it.
Note: Borderlands has several connectivity issues so do refer to firewall settings and port forwarding guide before attempting to join a game Lan Online.