I think you are both saying the same thing
The Phil Gordon approach just approximates your odds of hitting your outs out of the deck (50) -- 2% per out per card coming, so 4% with the turn and river both to come. It says that 4-outer on the flop has 16% chance of hitting as opposed to FRW's 20%.
In terms of calling bets, though, I think it is incorrect to call that 20 chips because you are likely to have to call off more chips after the turn. So I would need around 10-1 pot odds here (8-10% chance of hitting on the next card) which does not come around very often.
Of course, I seem to jack up opponents' implied odds here by stacking off when my set loses to that Ace-to-five gut-shot completion!! Grrrrrr
Some of the odds I use are:
Completing a flush on the next card - 4:1
Completing an outside straight on the next card - 5:1
Completing a flush or straight on the next card - 2:1
Hitting a set or flush on the next card - 3:1
Hitting a set or outside straight on the next card - 4:1