About a year ago Raven Software released the source to heretic.  It has subsequently been converted to a linux compatible version of the game.  If Raven were to simlply relese this same source under the GPL license, the game would be more usable to more people on more platforms.  There is a project called PRBOOM that has the doom I and doom II engine incorporated into it and is compatible with many platforms including Windows (NT 95 98 2000 ME XP), Linux (all distributions), Apple Macintosh, unix, Alpha and several others.  If the heretic source had the same GPL license the developers of this wonderful PRBOOM project say they would incorporate the game's source so that PRBOOM could also play heretic levels.  It is a shame to let a little license difference prevent this great old game from becomming more useful to more people.  Several people have signed a petition to encourage Raven to release the source under the GPL.  This petition can be found at the following web site: http://www.petitiononline.com/qhhgpl/petition.html

i don't know much about that licenses, but the doomsday engine at www.doomsdayhq.com supports doom1/2, heretic and hexen at a wonderful quality!

Technically JDoom and other GPL sourceports that have Heretic/Hexen support and/or use Raven sourcecode (such as ZDoom) are on sketchy legal ground.  A sourceport can't be GPL if the source isn't, so I can understand the PRBOOM guys not wanting to add Heretic/Hexen support if they want to keep their port GPL and fully legal.
Mercifully Raven has always looked the other way when it comes to this topic though (releasing the H/H sources under the GPL certainly would make things easier however).