Are you using Sins 1.05 or 1.09? Make sure you're also using the latest version of GF (if you are, the versionNumber in the map file will be set to 1). And these maps will not show up under "Custom", but should be among all the others.

There have been issues with people not seeing GF maps in 1.09. The recommended thing to try has been changing versionNumber 2. Some people say it doesn't work, others say it does, so to be honest I don't know if it will do anything for you

To update, someone posted that for a map to show up in 1.09 the browseName for it also needs to be set. I'm pretty sure you can do this in GF, but if not then opening the map file in Notepad, as well.