I was reading G&G 3.3, and G&Gr, and they aren't THAT different.
You could properly simulate the G&Gr system without going whole hog, by using the following rules-
Everybody has their con in HP (or 15 HP. Up to GM)
You must roll for Def each round.
Def(ense) = (the highest of your BAB or Base reflex save) + dex bonus
Armor counts for DR. Ie platemail is DR8, no boost to Def.
Every attack, the attacker rolls to hit, the defender rolls defense-
If the attacker hits (ie, modded role is higher) Defender taked dmg = the difference in the modified roles.
Ie, Attacker rolls a 8(modified to 15), defender rolls a 12 (modified to 13)-
defender takes 3 hp.
Otherwise, play as normal.
If you want to add one more layer on, then-
If the attacker beats the defender by 10+ points, it's a crit.
One more layer-
On a crit, Apply an effect from the called shot table
Weapons with bigger crit modifiers get bigger called shot table effects
(x2 gets a -4 crit, x3 gets a -6, and x4 gets a -8)