As Gwent updates go, patch 3.1 is a pretty substantial one. Not only does it add new content, like the new seasonal mode: Blitz, but it also overhauls several factions and implements more than 200 balance changes.
Among the new features in this update are the addition of the new Syndicate Leader, a new Special Arena Mode Weekend, new options when viewing cards, and more. The update also implements reworked abilities for several Northern Realm cards — and other card changes.
As for the balance changes, well… there are a lot, as mentioned above. The full list of changes are available in the patch notes on the Gwent site.