Geany is open source software, worked on by volunteers, if somebody wishes to do it, pull requests are welcome.
A quick look at the Gimp docs suggests it delegates the entire keybinding process to GTK. It would be a significant change for Geany to do the same. Also Gimp supports only GTK2, whereas Geany supports both GTK2 and GTK3. I'm not sure if the facilities are the same in both.