I've been trying to play the witcher 3 again but i had to reinstall it, so i did, but after it downloaded the download menu keeps saying "INSTALLING" and "moving 100%" but it does nothing and keeps loading.
I've tried to reinstall the game multiple times now but it keeps saying the same thing and i've even tried reinstalling GOG platform but nothing seems to help.

I hope my someone understands what i mean and knows what i should do.

You need to make sure you have enough space on the drive where your %TEMP% folder is -- move it if not -- and you absolutely need to be patient: the installation takes quite a while, depending on your hardware.

I've noticed just recently that installing a 250 MB game can take up to 2 hours. Why is that? Just a few weeks ago a game that small would install in about 3 second. Now I literally wait hours. Is the installer corrupted?

Your screenshot shows the galaxy client.
You don't need that to access your games.
Just download the normal game installers from your account, below the blue galaxy client banner.

https://www.gog.com/en/account

... This was a necroed thread, 5 years old...

"Installing" is a loose term for a lot of people. HOW are you installing it? Downloading and installing from Galaxy? Downloading the offline installers and running them? Where are you running the installers from - your PC's internal drive or an external one?

If I was running offline installers to install I'd suspect a drive issue for installers taking that long. How old is your rig?

If HDD: Check your event viewer for warnings & errors regarding such). You also might want to check its smart scores to get a quick idea of its health. Slow drives are a bad sign of a failing (or super-fragmented and full) drive.

Tried installing anything else that size other than from GOG? How quick are they?