Is there online multiplayer mode available?

Just Press Escape the game will get out of full screen and show a menu bar. Click on File and then Start a Network Game. You can then select if you want to be a host or a client.

how do i set that up?

I use Radmin myself.

I'll be more clear since for someone that doesn't know much about networks can get confused and I assumed the OP knew some of the basics. First of all, the store site says clearly: "Please Note: Multiplayer is available via TCP/IP LAN." What does this mean? It means that the game has multiplayer beyond a local match. The game is able to be played with another computer within the same LAN (Local Area Network) through the TCP/IP network protocol (you can go and check on your preferred search engine what are network protocols, but for this brief explanation that I want to give let's just say it is a way of connecting devices in a network. A LAN is just conformed by all the devices that are connected to the router in your house (that thing that gives "internet" to the devices in the house either by cable or wi-fi), let's say that you and your brother bought the game and both your computers are connected to the same router. You have to start the game and follow the instructions I have given above:

"Press Escape the game will get out of full screen and show a menu bar. Click on File and then Start a Network Game. You can then select if you want to be a host or a client."

One of you has to Create the game session and the other has to join by searching for active hosts. If the game finds at least one host it will show a list of every endpoint that it has found, if not it will just show an error. Once you select the joinable game, the games will synchronize and both of you will see the same screen once you are connected. I have tested it and works great.

Now you will ask: But how am I supposed to play online if you have just said it is only in my Local Area Network? Well, there are various programs that simulate that you are in the same LAN that other people throughout the internet. The one I use is Hamachi. I like the fact that it has "rooms" where you can join and everyone there will be considered in the same LAN. As you can imagine you have to coordinate the fight with your opponent in a forum / Discord / whatnot before joining one of these "LAN simulators" there is no such thing as a matchmaking server, there is no option to select "online" and then go to a "Quick Match". I hope this has answered your question.

I know only like 7 of us bought this game, but I think I figured out how to play over a network, I used that Hamachi gimmick that Bisonfan95 posted. Add me on GOG and we can exchange Discords, and we can try to get a few online games going, it would be nice to not play against the A.I.

Know what? Let's create a Discord Channel and post it here and on a new post so people that want to match up can join. Does that sound good? "Unofficial Mortal Kombat Trilogy GOG PC" could be the name.

Sounds GREAT!

I was able to connect with a friend using the TCP/IP in GoG Trilogy but after we connected the game would run at half speed. Anyone have an idea why it runs at half speed? Has multiplayer worked correctly for anyone? We used Radmin to connect.

Sounds awesome. Have you/someone else created the Discord channel yet?

Edit: It'd probably be easier just to use Parsec than to go through all of what was said before.

I have never used Parsec. How is it better?

Quick and easier to set up. The other person doesn't even need a copy of the game to play it with you.

As can be seen from the screenshots, the game only supports the legacy IPX protocol. This is the old protocol which is noe even supported in Windows XP by default (you have to install a separate Windows service for this). Not sure about the modern Windows, but in case you decide to play in multiplayer mode (via VirtualBox, perhaps), you will have to install the protocol itself first, then establish a connection through Hamachi, and then you lauch the game and select the online mode.

I tried playing on Lan Out of the box without installing funny things between a Windows 7 PC and a Windows 10 PC and "It just Worked" -Todd Howard

CAllMeNightHawk and me tried plating over Hamachi but was slow AF. IT DID WORK THO. I am trying now to set up a ZeroTier Lan Network to check how does the conection feel. Wish me luck.