but what kind of blades can you use? Can I just use regular razor blades?
No, you can't. Single Edge Gems use an entirely different blade, that is not at all compatible with the double edge blades you use in a DE razor.
See here, this website pretty much stocks the full line of available blades for Gem Single Edge razors:
http://www.shoeboxshaveshop.com/single-edge-blades.html
There are also "Ted Pella" blades, which can be bought here:
http://www.bullgooseshaving.net/gemptcoseblt.html