I went through this myself recently. Basically all auto flashes will work. Older flashes sometimes come with an integrated PC cord for use with the non-hotshoe Electro 35s. Look for a flash with a narrow base to clear the film rewind knob right next to the hotshoe. For example, my Vivitar 5200 cannot be put on a GSN but a Sunpak 411 can.
I was greedy and picked flashes with 4 auto modes. These usually require 4 batteries which means big and heavy. The Sunpak 411 is an absolute monster, I don't recommend it... The Vivitar 5200 is not too bad and I use it with my GX.
Another point to note is the ISO range of the auto mode. Some auto flashes only goes down to ISO 400. If you want to use the flash for fill that is a limitation (you're supposed to go 1 or 2 stops below ambient I think, but I am inexperienced in that).
The nice thing about the GSN is that the flash mode automatically set shutter speed to 1/30. I find that useful when it's really too dark and I just want flash to fire. I cannot figure out how to do that with my GX.