In order for Steam and GOG players to be able to share multi player, NMS would have to take on the added functionality of an internal friends list and in game name. As long as NMS relies on GOG or Steam for the in game name, as well as the friends list, then it would be on Steam and GOG to work together to share friend list and account info about thier players ... and tbo, I don't see that ever happening.
But interestingly enough if you see bases of Steam player and their discoveries. So this data is kept in centralized DB. MP is just a step away from this. Fortnite does it without any issues. I get the Epic is huge, but player base is also much smaller for NMS.
Well, if you're going to build a game that, as a standard function, uses a central database to store discoveries uploaded by players, adding a 'discovered by' field isn't a big deal, and all they're doing is just adding the player name supplied by Steam or GOG.
In order to add an account system within the game for storing an in game name, as well as a friend list, that would involve adding another database to work that out, and currently they are just using the Steam and GOG platforms to handle that for them.
So my take is, we won't see Steam / GOG crossplay anytime soon.