This is all my read on the situation only. I use 9atom and track
the mainline closely, but only casually track 9front.

The mainline Plan 9 distribution from Bell Labs is managed very
conservatively, from an external point of view. Both 9atom and,
later, 9front were started because that didn't work for the folks
who created those distributions, in different ways. It may be
fair to say both started as a way to get improved hardware
support into a publicly-accessible distribution quickly, although
there have since been many other changes in each system.

9atom often pulls in changes from 9front and the mainline. I
believe 9front watches the other two, as well (although I'm not
sure how closely). As far as I'm aware, the mainline only
considers changes in the others if they're explicitly submitted
as patches.

9atom is, procedurally, very similar to mainline: you submit
changes via patches, sources and everything else is available
via 9p, and so on. It also puts similar weight on compatibility
and similar concerns. 9front is a more radical departure for
Plan 9 (in some ways making it more recognizable for those
coming from elsewhere in the Open Source world), with a
Mercurial repository and a conventional issue tracker and the
like. It is also has a bit more of an experimental character
regarding changes to the system.

The GPL thing has no bearing on any of these distributions.

Anthony