GNAV was invented by Gordon Wansborough-White, who sadly died last year, a thoroughly nice chap if a little obsessive on the subject (Gordon had in an earlier life been an operational Sunderland pilot). His work on the top-nav competition, and with other bodies such as RIN was well known, and admirable.
VORTRAK is a great system, that was pretty much separate to GNAV, and basically uses two clever little pointers to give your position from a pair of VOR beacons. Jolly good idea, and makes a known task much easier.
GNAV was essentially based upon the assumption that you did virtually all of your cruising at about the same TAS (reasonably fair) and so the best way to fly was not to mess about with a whizz-wheel constantly, but to simply carry on your kneeboard a small table of corrections for different conditions flying at that speed. This worked very well.
In my opinion however, where Gordon let himself down was by adding onto this a whole and unnecessary system of navigation which really just became far more complex than the approach he was trying to discount as over-complex. This led to the occasional, and not entirely unjust, accusation that he had a solution looking for a problem.
But yes, I'd say it's worth looking at - ESPECIALLY vortrak, but if you dabble with GNAV I'd strongly recommend looking behind it and cherry-picking the useful bits, don't necessarily adopt it as a whole (or you'll end up as confused as I was!).
G