Heres the main site
https://www.playgwent.com/en
For those not in the know, here is what Gwent is:
http://witcher.wikia.com/wiki/Gwent
Who is in the Gwent beta? I just got in, got to level 6 and am currently running northern realms.
Current review of Gwent: Just put in a couple of hours, but I really really really like the changes they made from the Witcher 3 game to this standalone game. The game def feels more balanced. Nilf and NR are not amazingly more OP. There is a lack of spy draw spam, etc.
Lovin it.