Look here:
http://readandclick.com/store/index....=index&cPath=8
There are three kinds of certificates:
Email: This is the standard Zen Cart GC, nothing fancy.
Paper: This is literally a physical object for sale. When ordered, we go into the GC admin->Mail Gift Certificate, type in the amount and it generates a code. We write that code on the GC.
Printable: This was a pain to do. You set up a GC, and then use the attribute controller to create two attributes, which will be attribute type "file". It took a lot of messing around with attributes and it was a long time ago, but if you mess with attributes you will figure it out.