How to fix GPG in updater [duplicate]
Solution 1:
Run these commands in the terminal:
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 40976EAF437D05B5
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 3B4FE6ACC0B21F32
This downloads the missing GPG keys (as mentioned in the original error message) from the Ubuntu GPG keyserver.