Okay, so... I pre-ordered Cyberpunk on the GOG store but then regret the decision because I wanted to buy it on Steam instead. I sent a refund request to GOG and then received the automatic e-mail response. I responded it, saying that I was okay with the credit being returned to my wallet but so far there's no reply about that. It has been almost 7 days already and they keep leaving me in the dark. How long do they usually take to respond?

Gog has problems. They should remove the refund policy before Cyberpunk 2077 comes out. I can't say anymore because other parts of terms and conditions are treated the same as their flaky refund policy.

https://www.gog.com/forum/general/how_long_does_it_take_to_get_a_refund_1
https://www.gog.com/forum/general/how_long_does_it_take_for_refunds_to_be_handled_now

Try and reply to the support ticket and see what happens..

I already did that and still no response so far. Someone told me that they won't refund a game after 30 days have passed since the purchase so I want to solve this asap.

If you submitted your refund before 30 days there shouldn't be any problem even if 30 days pass (because they can see the day you submitted it) But since it is a pre order I believe you can refund any time before the game is released....Hopefully they will reply to you soon. They probably are busy with all the support tickets they have from the summer sale + working from home.