It appears to be DPI unaware.  

My fix for Gnome 3 at 200% on Debian Bullseye:

1. Create a copy of the .desktop to home directory
# $ cp /usr/share/applications/x2goclient.desktop ~/.local/share/applications/

2. Change exec= in ~/.local/share/applications/x2goclient.desktop
Exec=env QT_AUTO_SCREEN_SCALE_FACTOR=1 x2goclient

I found this recommendation in https://doc.qt.io/qt-5/highdpi.html
"To get an application designed for low DPI values running on high resolution monitors quickly, consider one of the following:"
"set the QT_AUTO_SCREEN_SCALE_FACTOR environment variable to 1."

Thanks to akik in the #x2go IRC channel for pointing me in the right direction.