Thank you for your answers. I got it working today, and there were 2 issues causing my problems. First, the csync voltage was too high. The input impedance on the GBS-8220 is about 500 ohms. By putting a 570 ohm resistor in series it knocked the voltage down to around 2v. The other issue is that the GBS-8220 appears not to be able to handle 50hz. It will only sync to the signal when the Amiga is in NTSC (60hz) mode.
I had this working a few months ago with an Ambery AV-1 converter, and it could do the 50hz modes as well, but now it doesn't work. I think that this converter is dead. I'll have to get another one and try it again.