Are you sure the most common mistake is people save their maps under
Steamgames \ Steamapps \ common \ Sins of a Solar Empire Rebellion \ Galaxy Forge
It won't show up if its in there and will do just as you are saying
Load you map in GF choose save and double check the current path in the upper window if it is
Steamgames \ Steamapps \ common \ Sins of a Solar Empire Rebellion \ Galaxy Forge then click on
Libraries \ Documents \ My Games \ Ironclad Games \ Sins of a Solar Empire Rebellion \GalaxyForge
and save your file there, if that don't do it