HelenOS[1] is in C and has a GUI. I don’t know how many people participated over its (quite extensive) history.

Axle[2] is a one-man project with a GUI that the author has been gradually transitioning from C to Rust.

Among C++ projects, I think Essence[3] also merits a mention.

[1] http://www.helenos.org/

[2] https://github.com/codyd51/axle

[3] https://gitlab.com/nakst/essence