I bought The Witcher 3 GOTY Edition a few days ago and I installed and have been playing for 3 days now and suddenly today I see the game not showing in my library both on the gog site and the client. I even tried redeeming the key again since I've kept it but it's already claimed so I don't know what to do right now

Did you buy it on GOG or on G2A/Kinguin?
Because if it wasn't on GOG the key might just be nullified because it was fraudulent.

Never buy from G2A, that was your problem. Only buy gog games from gog.

No, it can take a while before GOG notices.
If you can't see it in your library, it's gone and you should contact G2A.
And really, don't use them.
They're rarely cheaper than the real stores, but a lot more fraudulent.
For the most part only GOG sells GOG keys, the only legal reseller I know of is Humble and then only for W3 as far as I know.

There's no way of getting support from gog for this issue? seriously?

Nope you're most likely screwed, as the other guy said, G2A is infamous for selling keys bought by stolen credit cards.

Why should they support you? You bought from a fraudulent store.

You can always try, but I'm 99% sure they'll tell you the key was revoked because of credit card fraud.
Since you paid extra for G2A Shield you should also shoot them an e-mail.
I doubt it'll be a fun experience though.

Should I contact GOG support just in case?

You could try but it would basically be a waste of time, basically when you get games pulled from G2A it's about 99.9 percent bound to be fraud so you aren't gonna get the game returned to you. I would just try what I said, refund from G2A then just buy here.

Ok, I'll try and see what happens

Good luck man. :/

Good luck!

P.S. Summer sale is just around the corner. You will most probably get good deals on GOG itself.