I've downloaded the Witcher 3: Wild Hunt on the gog galaxy app and when I select play, the game doesn't start and it tells me to close the programme...? Does anyone else have this problem and how can it be fixed?

Did you try starting the game directly from where you installed it?
Not exactly a solution to your problem, but a workaround.

I started it up from where I downloaded it (the gog galaxy app) but it keeps on saying that it can't load?

Post your system specs (short version).

I think it's fine now, I've moved stuff and downloaded more files, it now says it is released in under an hour -_-

Ok,no worries.

I take it back, it's still not working -_-