Hi, Andrew.
In windows, Gusek uses monospaced Courier New by default for edit and output windows.
You can see in SciTEGlobal.properties:
font.monospace=font:Courier New,size:9
You're running Gusek under wine, maybe your system don't have the font?
Is the ttf-mscorefonts package installed?
By the way, you can replace it by your preferred fixed-pich font.