A volume of 1 gal equals 231 in3, see
the density is in the range of 10 000 kg/m3 = 624.28 lbm/ft3, so we get
m = ρV = 624.3 lbm/ft3 × 0.25 × (231/123) ft3 = 20.86 lbm
A more accurate value ρ = 848 lbm/ft3.
For the air that density is about 1 kg/m3 = 0.06243 lbm/ft3 so we get
m = ρV = 0.06243 lbm/ft3 × 0.25 × (231/123) ft3 = 0.00209 lbm
A more accurate value is ρ = 0.073 lbm/ft3 at 77 F, 1 atm.