Glevolution is supposed to be a diffrent "Distribution" of Glest, including more stuff, like translations + mods, which one basically could just download and use with the "Normal" Glest, but whatever ... And both are licensed under GPL, at least the source codes
Concerning the Artwork it seems to be up to the individual contributor, some don't have any license on them, others have a reference to GPL, but do not contain the license itself ( ARGH!!!! ) ...