We are continuously working on optimising the game. It is possible that along with our optimisation tweaks and game engine updates in the future the performance on these video cards will improve. However, we can't promise that this will solve the issue as it is related to old Radeon cards that have been not receiving updates since 2015/16.
Additionally, the following workaround may help:
1. Run GOG Galaxy.
2. Click Gwent on the installed games' list on the left side.
3. Press MORE button, select "Settings"
4. Check "Add command line arguments". Input field will appear.
5. Into the input field type in the following text: gwent.exe -force-feature-level-10-1
6. Launch the game.