The Sunday/Monday options is indeed a setting, but it uses an xsetting interface that I have no idea how to use. The program does look for an xsetting value for this, but not knowing how to change it doesn't help.

Terrancegf made an extremely simple change and made it available to anyone who wants that option. I don't think Terrance intended this to be a permanent change to GPE, but I am glad he posted it here.

If anyone knows how to use xsettings and can change the Sunday/Monday option with that, please post here so people can see how to do it.