yestok: An answer to a review saying that multiplayer is inaccessible.
TDLR:
Use the Galaxy rollback feature for now to revert to version 1.0i to play online multiplayer and use crossplay on the GOG version.
https://support.gog.com/hc/en-us/articles/4419630535057?product=gog

When you just had bought this game and wanted to install it either via your latest offline installer or just pressed the Install button on Galaxy you might have encountered a problem when going to the multiplayer option on server browser. Either all servers show a red cross at the ping section or you see no servers at all. When trying to connect to those red cross labeled servers the game would give you the message "Client version or data does not match server's." You are on Version 1.0j. So you might think you are not able to play multiplayer on those servers. But that's not true.

On GOG release date (9th March 2023) there were two versions released at the same day shortly after another. The first one (1.0i) had and still has full online multiplayer and crossplay support with Steam and you were and still are able to join all servers. It is on par with the current Steam version. GOG Galaxy let's you downgrade to previous versions of the game. The second and latest (1.0j) update (that gets chosen for download) is a newer version that Steam users haven't recieved yet. Therefore their servers run on an older version of the game as the Steam version wasn't updated since 20th January 2023. In the future Steam, GOG versions will very likely run on the same version as they probably both get the same updates.

But as of now you need to downgrade your GOG installation to 1.0i in order to have the same version number as steam users and play together. Use Version 1.0i in the dropdown menu.
Guide:
https://support.gog.com/hc/en-us/articles/4419630535057?product=gog

Alex0007lolpvp: Have anyone found solution to this? I'm looking to buy this game, but i'm afraid that multiplayer is not working

Purple44: Hi guys, you guys do know Bugbear finally release an update today that make GOG and Epic PC versions of Wreckfest compatible online with Steam version? Took over a year, but Devs finally got it done.

jannesBB — 01/03/2024 2:23 AM
The update is still in the pipeline but it has been delayed for reasons beyond our control and I'm sorry to say that's all I know.

I can't post Steam link:

PC Crossplay Update OUT NOW!
Get Ready, Wreckers & Racers: PC Crossplay is Here

Howdy folks,

We're thrilled to announce that Wreckfest now supports PC multiplayer crossplay. For the first time ever, you can join the same lobbies with your comrades across Steam, GOG, and Epic Game Store platforms, uniting the playerbase. So from now on, it doesn't matter where you bought the game – you can now smash and crash your way to victory with all your friends and foes!

And wait, there's more. We're admittedly big fans of crashing here, but cars and of course not servers, so we've also gone and given the engine under the hood a fresh change of online oil in an effort to further improve server stability and make sure the servers can handle all the heat while you embark on your wrecking rampage on all platforms.

Update now and teach your buddies from GOG and Epic Store how to bend metal!

*************************************************

Can anybody confirm that they can now join a stock Steam PC WF server today with current version of GOG Wreckfest?

No downgrading to version 1.0h.