I spent an evening trying to get Terminal Velocity multiplayer working over a LAN a month or two ago, so I thought now that GOG has this game I would share the solution I found!

I'm running the version purchased from 3Drealms, on Linux, through DOSBox Game Launcher v0.74. I expect my solution should work for GOG customers, but if it doesn't please let me know and perhaps I will try to edit this post to reflect a working solution. If GOG has already included a guide or utility for multiplayer, though, then you can ignore this thread. :)

Lets get started!

GOG should have included a dosbox configuration file you can edit to start multiplayer. We want to edit the config file that launches SETUP.EXE.

At the bottom of the file, there should be a series of commands that are executed in Dosbox upon launch. Right before SETUP.EXE, we want to insert a new line. This line will be slightly different for the host and the clients.

host:
ipxnet startserver 9600
client(s):
ipxnet connect [host ip] 9600

Note: 9600 is the port number, and may not be strictly necessary.

Alright, that's it for editing! You can now launch SETUP.EXE with the Dosbox config file that starts or connects to the server.

Host:
Choose "Network Game"
Specify number of players
Choose "Launch TV"
Client:
Choose "Network Game"
Specify number of players chosen by the host
Choose "Launch TV"

Firewalls permitting, TV should now start!

The host selects "Play Game" and sets up the game type and map. Once this is done and the all players have chosen their ships the round will begin. (The host is also a player)

Hope this helps!