I am trying, successfully to debug it through gdb:
gcc -g test.c -lds

This is not a GDB command, but a GCC one, building a file named test.c, adding debugging info and linking libds.

Running under GDB would look like

$ gdb a.out
...
(gdb) run
...

BTW, note that Geany doesn't have any builtin debugger, but three plugins: debugger, scope and geanygdb, so it's unlikely to be a Geany issue.
And code 127 usually means "program not found", so maybe you just don't have GDB installed?

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