Based on the few B&W photos I could find of Gabby's Jug, it really was a toss up. So I opted for the Gray under surfaces. Later P-47s in the 56th (such as Schillings "Hairless Joe) do show NMF undersurfaces. Also check your references for the fueslage invasion stripes. The Photos I used for reference showed only the white stripes outlined in black on the fuselage, not the full black bands around the fuselage.
Here appears full black & white stripes along w/ NMF lowers
And this one has what appears to be gray lowers along with the outlined white invasion stripes
F is for FIRE, That burns down the whole town!
U is for URANIUM... BOMBS!
N is for NO SURVIVORS...
- Plankton
LSM