gEdit Project Manager is a gEdit plugin that allows grouping files into "projects" (a.k.a. "filesets"). It includes all the basic functionality needed for this kind of taks, and well... much more. What started (v 0.x) as an attempt to provide a solution to "i really need to group these files together" evolved into (v 1.x) a plugin that completely transforms gEdit into a multi-view-per-file editor packed with cool document-navigation features.
Versions 1.x to be released in a couple of days. Stay tuned.