Alright so there's currently 2 ways to do it:

1. Host/Join multiplayer via LAN
2. Use multiplayer client

You can either port forward or play over a VPN/P2P network tool.

If you don't have a static IP or don't know that that means, skip to VPN host, otherwise this comment on SWGB heavens explains nicely what ports you need:
https://swgb.heavengames.com/cgi-bin/forums/display.cgi?action=st&fn=1&tn=10588&st=3#post4

Then multiplayer should work for you.

This is the way I play the game with my friends.

Download something like hamachi, zerotier or radminVPN (I use radmin). Setup your private network connection, invite friends to your network.

Then download IPXwrapper from here:
https://www.solemnwarning.net/ipxwrapper/

Or from github here:
https://github.com/solemnwarning/ipxwrapper/releases/tag/ipxwrapper-0.7.1

It's a zip file that will help you use the deceased LAN functionality of SWGB. Open the zip with 7zip and copy all files ending with .dll to your SWGB folder, usually located at:
C:\Program Files (x86)\Steam\steamapps\common\STAR WARS - Galactic Battlegrounds Saga\Game

Then if you have 32bit system, run the directplay-win32.reg, otherwise run the 64 thing. This will inject some stuff into your windows registry, which will setup IPXwrapper as a connection type for SWGB.

Now that you have both radminVPN (or alternative) nad IPXwrapper installed, run SWGB.

When you open the game and click on multiplayer, a new connection type named "IPX connection yadyyada" appears :

You can use this conenction to play LAN over hamachi, radmin, zerotier, you name it. Works for me, I'm using this all the time. If you host a game, anyone on your VPN network should see the game in the IPX connection type window in SWGB. If you don't see it, copy the host IP and try to enter it to a window that appears when you press join.

IPX wrapper is windows only, but I don't think SWGB runs on any other OS without an windows emulator. Let me know if I'm mistaken.

Using a multiplayer client has only 1 advantage over LAN hosting, and that is playing with total random people, which can be fun if they're not a tower rushing.

You have two viable options :

1. Gameranger (https://www.gameranger.com/)
This worked for many steam games flawlessly until some ♥♥♥♥♥♥ broke something. Now steam games don't work with gameranger anymore, and you have to move the game files away from the steam folders for it to work. Do not put large files/folders on your desktop, that will criple your PC performance.

To play you just click host and then invite friends, or your friends can join when clicking on your username in gameranger.

To play Expanding fronts you have to manually rename your battlegrounds_x1.exe to battlegrounds_x2.exe, and your battlegrounds_x2.exe to battlegrounds_x1.exe. This will make it so that gameranger launches Expanding Fronts instead of Clone Campaigns.

2. Voobly (https://www.voobly.com/)
Voobly faster to setup than gameranger because it actually works, and is ususally recommended to use to play SWGB nowadays. There even is an Expanding Fronts mod and exe patch to use while playing.

To play, join the SWGB lobby (use this link if you can't find it : https://www.voobly.com/games/view/SWGB-Clone-Campaigns/Voobly-Lobbies)

1. Is this legit ?
I personaly tested everything I listed in this guide. I run a full system scan each week and haven't encountered any issues.

2. My antivirus flagged something as a virus
Nothing I mentioned here is malicious. If your antivirus marked something as virus or malware, it's a fake positive. If you use McAffee, switch to a different antivirus. I had McAffee in the past and it kept marking random files as viruses even tho they weren't.

If game throws an error while launching, run it in windows xp or 7 compatibility mode, close it, and then run it again.

If you can't even launch the game in the first place, open your antivirus and exclude the whole SWGB folder from detection. Then delete the folder contents and use the steam validate files function to redownload files. When done, you should be able to run the game.

There are mods out there for widescreen and 60fps, I use Expanding Fronts but there is also the All-In-One patch I think. Give it a try.

Thanks for reading, if you encountered any problems, let me know in the comments.