Can GOG and Steam versions play online together?
Can Windows and Mac play together?
Can a Steam user on a Mac play with a GOG user on Windows?

Thanks in advance :-)

As long as you use the same version (1.3.2.1 is the current Steam version) and use the "Join via IP" you should be able to play together. Also, you need whoever is hosting to be able to host (open ports on PC and whatever router the host is using) and you need the actual IP-adress, use a web service to check if you don't know, there are plenty.

If it doesn't work, it's likely that something isn't set up properly.

Lovely! Thank you for the reply.
I might have to pick up a copy of this for my (Mac) friend then :-)

Yep, I've used both the GOG and Steam version to play with both GOG and Steam players. The only issue that generally arises is that Steam usually gets new patches a day or two before GOG does, so there may be short interruptions in the future, because the dev team isn't quite finished with Terraria yet.

If you run into a connection problem, it's almost always due to port forwarding issues. The host will usually have to look up specific instructions for their router model.

There were some issues with the 1.2.x releases being late, so it's very good to hear that those have been resolved!

A fix for Linux and Mac server was not included yet in the GOG version (gog_terraria_2.5.0.8.sh):

Official forum changelog and the ofiicial dedicated server fix.
The problems below were fixed by replacing the provided TerrariaServer.exe with the one from the "zip://Dedicated Server/Linux/TerrariaServer.exe"

And with some more testing, the Linux server does not work, the dedicated server hotfix only partially works, as noted by other users in the changelog thread above.

As of 1.3.3 the GOG version Linux servers do not work here.

I cannot get into the server by hosting a local game from the client or either with the dedicated server.
The problem is with both the 32 and 64 bit servers.

The server seems to be running, as the all the server commands seem to work.
The time also did not progress, but that's what I'd expect as there are no players connected. Then I tried the Linux server on another machine, and there the time did progress, but no-one was able to get into the world nevertheless.

All connections to the server hang indefinitely.
Server log just gives: "<ip>:<port> is connecting..." and nothing else.
The client hangs in "Waiting for server" or goes as far as "Receiving tile data: <xx>%". The percentage got up to 40% on one of the worlds I tested (and once up to completed on the other machine).

I tested with newly generated world, and all the old ones on the machines. Server did not work, but the single player does.

The Steam 1.3.3 version linux server and the GOG Windows server seem to work.