mechmouse: The arrival of Epic has, as feared, took a chunk out of GoG. Many devs/pubs are using Epic as their "alternative" to Steam.
I am wondering though, why would any dev need Epic as an alternative to Steam? It most probably can't be that they get much more sales through Epic (so it would make sense for them to release some of their games on Epic, instead of Steam), so does Epic have some features or tools that Steam doesn't offer?
I am pretty sure most devs/pubs would most likely want to release their games on their own store so that they would get all the money from the sales, but since that is not feasible for most publishers, the second best thing would be that there is one big PC digital store through which they can sell all their games. At the moment Steam is pretty much that.
Whether they'd release the game on some other store as well, in addition to Steam, that makes sense only if they believe they get enough extra sales (or other perks) from that other store, so that it makes sense to use resources to publish the game on that other store as well.
Those extra sales come only through customers who, for some reason, don't want to buy (at least some of their) games through Steam. For GOG that is pretty obvious: DRM-free installers. At least I can't think of any other reason to buy some game, which is released both on Steam and GOG, from GOG instead of Steam, if it wasn't DRM-free on GOG. For instance, I bought Skyrim from GOG (not Steam) for exactly that reason. If the GOG version had DRM and required the Galaxy client in order to play it, at least I can't think of any reason why I wouldn't have rather bought it from Steam instead.
I've yet to figure out, why someone would buy a game from Epic Games Store, instead of Steam, if the game is available on both services. People go to EGS to get free games (that EGS still offers regularly), and Fortnite, as Fortnite is not available on Steam. Well, those times where EGS had a time-restricted exclusivity to some new games, maybe some fell for those as well, not sure...