I have the Metz 20 C-2 too having bought it after extensive research. It is nearly the same size as the GRD but because it only has 2 AAs to power it, it is not heavy and the balance of the camera is reasonable with it fitted. It also has a bounce capability, a locking foot and 2 power (aperture) settings. For reliable results it is best to set the camera to manual and select a shutter speed that you can hand hold and an aperture near to either F2.8 or F5.6 as selected on the flash....you may need to stop down slightly for best results. A consequence of the 2 power cells is that it can take a while to recharge so if I know I'm going to be taking flash photos I fit newly recharged cells and take some spares.
A 2nd hand alternative could be the Olympus T 20 (I think the T 32 would be too big/heavy but may be not). I had a T 20 years ago and it was a good gun and the 'flat' design would probably sit better on the GRD than the Metz - however, no bounce head (the T 32 does have a bounce head).