I thought this is a good idea and actually tried.
After pressing the Donate button I'm able to donate to two sub projects.
But then on either one I can only chose a "monthly plan".
Guys, please don't do that! Just allow me to pop 20$ or 50$ to your project (which I love and sometimes use) but don't force me into a subscription like thing, which I really detest.
Maybe it's me and I was just too dumb to find the correct button. If somebody has a link to a one-time donation for the GIMP project please feel free to post a reply.