So, I tried to register on Gameflip throug Google (since I don’t have Facebook) but in the moment I press the Button a small window for the authentication appears and one secound later it disapears again. If someone could help me that woould be really great.
That is unfortunate and I am sorry to hear that happen. Sadly I can not help much in this. Have you submitted a ticket to Gameflip support yet? If you can that would be great so they can get to you on monday. If you arent able to just wait til they get to the forum and help you here.
While you wait and for a faster response if you have any other information to add for them to view that would be great ( screenshots if avaliable) that way they can help you get your issue fixed at a faster rate,