There is a concern about voltage. I currently use an old Canon Speedlite 199A on my G3. I believe there are several websites that have lists of voltage levels of various flashes. Here is one link I found:
make that two:
It's not the best one but a search under voltage, compatible, flash and camera would probably turn something up.
There are cases where damage has occurred. Please research it before you try.
Canon Pro1, also G3, TCON-17, Lensmate