Just to clarify, ID has released the source to Quake, Quake II and Q3A under the GPL. Commercial projects can be developed on this code (i.e. free engine, commercial content). Those who wish to develop commercial engines from the Quake code (e.g. Half-Life engine) will need to arrange a licensing deal with ID.
For future versions of Tremulous to be commercial, this would require a complete rewrite of the current Tremulous content unless all of the authors can be contacted and agree to commercial licensing of their contributions.
Commercialisation will also escape the point of the project; to create a free game.