This is also happening whilst running Skyrim and has the same effect of minimizing the game and freezes it when auto update kicks in does gaming mode prevent avast from taking priority or does it just stop the notification popups.
Gaming mode should prevent avast from taking priority and dropping full screen apps to the tray if it is to work correctly but does it?
If it’s anything like my issue (slow computer, eventually crashing games even though my computer should be able to run them at max settings), then it’s the “deepscreen” feature that Avast added in one of their updates. Go to settings → antivirus, then deselect deepscreen. You’ll see a HUGE jump in quality.