You might be able to write single sided (400k) disks on your IIgs, though I'm not sure which software you'd use.  Copy II Plus does support single sided disks (in fact, I have an original Copy II Plus 3.5 disk that is single sided), but would only support images in its own format.

Hmm.... I have an SE/30, and it will write single sided.  I need to make some disks for my Mac 128k anyway, I could try to image them on the IIgs with Copy II Plus....