I don't think it will be abandoned, but I expect slow development from here on out - because it's pretty much feature-complete already. Although there are still lots of things that could be improved, I think Galaxy is actually pretty good currently. For most things, I'd prefer Galaxy over Steam.
And sure, it's possible that it will be abandoned - but the nice thing about Galaxy is that it is optional anyway.
A couple of things I like better about Galaxy compared to Steam is that Galaxy has better options for displaying the game list, such as listing the game names as plain-text under the box images. And Galaxy's auto-update system is better - it can update automatically if I want it to, and updates can be disabled if I want; whereas in Steam updates are unavoidable; and it is difficult to stop them from being automatic. Sometimes games start updating even if you have explicitly told Steam not to do that.
As for what's better about Steam... for me it's basically just the workshop. The Steam workshop is a huge feature which is very valuable. But I don't think Galaxy will ever have that feature, because it requires a larger community, and presumably large amount of work from game developers to use an appropriate API. GOG is at a significant disadvantage in being small. Developers write their games to use Steam features (such as achievements and the workshop). Galaxy features in games are an afterthought - if at all.
