If you haven't received rewards on your GWENT account, despite claiming them in your Twitch drops inventory, make sure that you have the correct GOG and Twitch accounts linked.
The process below is to remove the current link between the accounts and establish a new one:
1. Go to https://www.playgwent.com/en/twitch-drops/unlink, log into your GOG.com account and unlink it.
2. Log in to all of your Twitch accounts that you could have connected to the GOG.com account. In Twitch account settings remove the existing connection between GOG.com and Twitch accounts.
3. Go to https://www.playgwent.com/en/twitch-drops/ and click "Link accounts" to establish a new connection.
4. See if the rewards are added in-game.