Hey! Do not listen to the people saying you can't find great multiplayer experiences on GOG.
The only titles that I've personally played multiplayer lately (even though they are not RTS) are:
1. Diablo with the DevilutionX mod installed
The mod helps the game run on modern systems, including modern resolutions, and offers some quality-of-life features.
Link:
https://github.com/diasurgical/devilutionX There is also a DevilutionX discord server there are channels where people sometimes gather for multiplayer games.
Link:
https://discord.gg/devilutionx 2. Heroes of Might & Magic III - with the Horn of the Abyss & HD mods installed
Link where you can download both the Horn of the Abyss mod & the HD mod for HOMM3 compatible with HotA:
https://www.hota.acidcave.net/download.html The game has a HUGE multiplayer following, there are games going all the time.
Also, as someone mentioned before, GameRanger supports a HUGE amount of games and you can use it to play multiplayer. The only game I recently played personally through GameRanger with a couple of friends was Empire Earth, but it has a huge selection of all kinds of genres.
Finally, if you are ok with using the GOG Galaxy optional client, there are many games that support multiplayer natively through Galaxy. I've recently actually finished playing Divinity Original Sin 1 with a friend from another country through GOG Galaxy (I have no clue if DOS1 multiplayer actually worked without the client, never tried).
Sure, many games released on GOG might come without a native multiplayer framework/solution/etc but don't let that discourage you! There are a ton of community solutions available, especially for older games!