Some people try to downplay this news, but this is a huge deal. I can't count how many times I heard that it would be great if we had only one launcher, which is connected to every other libraries. GOG Galaxy 2 will be just that and it will be great if they can pull it off. Obviously if you want to use a specific feature of that other platform, you have to download their client, circumventing that would be impossible. But I don't care about achievements and stuff, so I will be happy if every game would be available for lunch from GOG Galaxy.
It really comes down to execution. If it can manage my Steam/Origin/Uplay shit for me and launch games without me having to fuck around with those every time, it'll be very interesting. If all it does is launch Steam for me and I have to use Steam as normal, then it's useless. Will be very interesting to see what it's like.