I have a Raspberry PI running their latest OS (9.1 Stretch)
I can't seem to get full screen. If I set x2goclient to Display Fullscreen, it starts up and works fine except that it is a window on my screen (having a headline with minimize, etc. buttons). Then when I exit, it changes the config to Custom 1920x1078.
In fact, I just noticed that if I select "Fullscreen" and then click "OK", when I go back in, it's changed to Custom 1920x1078. Same thing with Display Use whole display.
Nothing I do will make "Fullscreen" stick.
Not having any problems on my LinuxMint box.