If memory serves me, progs 1.06 source *was* released under GPL, the big problem being prior releases (specially 1.01) which were used as base code for a number of well known mods (IIRC threewave CTF falls on this situation, among others).
EDIT: derp, just read what leileilol said some posts before this one, it's exactly the opposite - 1.01 is the GPL'd version, 1.06 source being in the gray area.

I suppose one could pick the 1.01 code, apply some patches to turn it a drop-in replacement to 1.06.