I tried installing the multiplayer files but when I try to start up the game, it crashes after a few seconds. Are there other files for GOG version?

I have the same problem. Did anyone find something to solve this?

according to this it won't work:
code.google.com/p/tlmp/wiki/GettingStarted

maybe someone with programming knowledge could help?

If i remember rightly, Torchlight 1 was only singleplayer (but fantastic). Some people worked on getting a multiplayer mod out, but never managed to finish it/get it working properly (i think there was some footage on youtube etc..) . Runic released Torchlight 2 that while later with the multiplayer, so for playing with friends the second one would be the one to go for. Hope this helps... and i could be wrong! or confused! 1 and 2 are well worth playing.