Will Gog Downloader be removed now when there's Galaxy?
I do NOT like Galaxy. Like, at all. And I don't care if it's just a beta. Why did Gog had to go and become Steam, Origin and UPlay to name a few..?

It's stopped being supported, which means that if they eventually change something significant in the background it will break, and it will stay broken. Until then you can keep using it.

Because it's not?

The downloader will not be removed intentionally, but future updates to the protocol may have it break.

Hopefully, a Windows or OS X (I don't know what you're on) developer will build a third-party downloader for you to use when it finally does break.

Because forcing your customers into using something they don't like.

How are you forced to use an optional thing when the download links are still usable via the site? And will remain so?

Because there's about 10 000 000 links for some titles.

A huge exaggeration on your part, Galaxy is no more and no less optional the downloader is/was when it released. It also does the same thing as the downloader (download installers and extra content), just not as well yet granted. It's the evolution of the downloader.

Also, there's always server errors for me.

Valid excuse though for using the downloader over Galaxy right now while it's in beta, you other ones... not so much.

Just to make sure you realize that you can download the non-Galaxy installers too with Galaxy, just like you could with GOG Downloader. I didn't face any issue doing that, it downloaded the installers fine. So far I've used Galaxy as the installer downloader, not for installing nor running the games.

I actually slightly prefer Galaxy for that purpose, because with Galaxy I can initiate the installer downloads directly from within the client. With GOG Downloader, I have to log in to GOG both via the web browser and the client, and then initiate the downloads from the browser, even though it is the client which does the downloading.

Steam, Origin and UPlay don't offer any optional ways to download or install the games (GOG offers the ability to download separate installers, either with the client, or even completely without), so claiming that GOG is now similar to those is incorrect.