My friend bought BG2 complete (BG2 plus Throne of Bhaal) on GOG to play with me, and I have BG2 complete purchased in the store a few years ago... unfortunately the version purchased on GOG doesn't seem to be the same as the regular retail version and we can't play multiplayer together. His game appears in his computer as simply Baldur's Gate II: Shadows of Amn while mine is Baldur's Gate II: Throne of Bhaal. Because of this the games won't recognize each other either through Hamachi or Gameranger and we can't play.

Is there a way to work around this? Thanks!

What error are you getting when you try to connect?

It simply says it can't connect to the server, no matter which of us tries to host.

Gameranger won't acknowledge the location of his game, and since it thinks he has BGII: SoA and I have BGII: ToB, it won't recognize that we even have the same game to let us attempt to connect.

Try connecting directly without stuff like Hamachi. I play BG Coop with a friend of mine and the only way we got it working was setting up a DMZ in our router and directly connect via IP.

Try this:
Close everything Hamachi/GameRanger related.
One of you (the Host!) configures his router as a "exposed host" or "dmz". That basically means that you disable your router firewall. The option should be somewhere under "Port Configuration/Forwarding" or similar. Depends on your Router.
Connect to the IP of the Host. (www.whatismyip.com)

This worked for us. And we even got different language versions of the game (Retail Complete BG in German+ GoG).

Oh and don't forget to turn exposed host/dmz off after playing.

Will this work even if our computers think they're different games? His game says it's "BGII: Shadows of Amn" even though it also has the Throne of Bhaal expansion installed, while mine thinks mine is "BGII: Throne of Bhaal".

When we get the chance we'll try this, though. Thanks.