An interesting attempt, yet so half-baked. Piece by piece, it looks rather well made, and doesn't seem to have great flaws, but… it's not quite obvious what G&G is supposed to be, but it noticeably falls short in any case.
- If it's supposed to be very generic-D&D5-compatible for the ease of conversion, what's with CoB added to AP and with spells?
- If not, why import everything in the dull generic state?
Bad import: weapon qualities. While simple/martial weapons concept is not so bad in itself, d20 lists are quite obviously "gamey balance" based and make no sense beyond that (see where bows, crossbows and slings are, and laugh).
Why not use what's already here?
- "armour piercing weapon" is not a new idea even for D&D (back in Birthright, crossbows at short range). And there's already penetration rating by damage type…
- If you determine locations for wounds anyway, why sometimes use generic AP, much less by counting from location-specific values? The result may also "jump" due to Db with varying multipliers dependent on the categoryb added to the rest of AP. Then it's mixed up with encumbance to make things more roundabout, calculating weight from AP.
- What's the point in using CoB x30 if CoB is already Con/10?
From what's there… attack DoS being added to damage looks like a good idea, but needs some refinement, mainly for tying to weapon types/qualities. Also, this means ability contributes to damage twice (directly and via attack roll), not sure if this makes things better or not overall (if not, two-weapon fighting will need overhaul too).
Wound Rating needs refinement - at very least, to be directly countered by some function of size (as in, "like birdshot to the elephant"), and later tied to weapon qualities.
"Advantage/Disadvantage" reroll mechanics works for strategy, but IMHO is too clumsy for RPG in general, and when everything else is done via stacking adjustments, "advantage for this but disadvantage for that" it's just a pointless side-step.
Likewise, flat "always 5%" critical success/failure borders on slapstick in d20 and is just too clunky for d100.
Of course, if d20v5 bugs are not features, why not improve a little - e.g. make armour work differently from evasion, or use more than 6 attributes (much like in certain other d100 games)?