I've got a blocking Ubuntu now. In the default configuration, the entire GUI blocks, which is pretty amazing.
Blocking the VTE terminal, setting the debug console to non-VTE, and using an empty program without any output, compiled without -g, helps *a bit* - Geany uses only ~98% CPU time instead of 100%, you can switch file/debug tabs, and see the debug console: some of the startup gdb commands are executed and their output is displayed, very slowly, but the breakpoint at "main" is not reached, or the gdb output from hitting it is not displayed, at least not in any reasonable period of time.
The next step would be to set some debug output in spawn_read_cb() and see what happens there...