Gnome-Keyring of current Debian stable always memorizes decrypted GPG keys for the complete session (i.e. until logging out). This is far too long and pretty dangerous and currently there is no way to change this in any settings GUI.

Here is how to do it on command line:

gsettings set org.gnome.crypto.cache gpg-cache-method 'timeout'
# values are 'session', 'idle', 'timeout'
gsettings set org.gnome.crypto.cache gpg-cache-ttl 300
# value is in seconds
#gsettings set org.gnome.crypto.cache gpg-cache-authorize false
# no idea what this means, my XFCE4 desktops have it set to false