Assuming you are talking about Blood:Fresh Supply (B:FS)

If you just want to start a LAN server without playing around with GoG Galaxy.

Get the IP address of the computer that is going to act as server. For this example we will just say 192.168.0.2
Launch B:FS on both Server and Client

On the server
Tap the tilda key (~) at your workstation acting as a server and type in the command spawnipserver
This may prompt a firewall action from windows for port 5029

On a client workstation
Tap ~ and type in the command connectip 192.168.0.2 (whatever the IP is of your server)

You and your clients should then be in a pre-game lobby. At the command prompt (~), type map 5 1 to start the bloodbath map The Stronghold, or whatever else you want to play.

map command format can be either:
map mapname or
map episode# map#

Note that this should also works across GoG and Steam clients as per NatsuruNat's instructions