Your equation is perfect.
That's exactly it.
The constant 'G' for SI units is 6.67 x 10⁻¹¹ N m²/kg² .
Also, 'D' is squared.
(Could one of these be your whole problem ?)
M₁ = Bob = 76.2 kg
M₂ = Peggy-Sue = 72.1 kg
D = 8.54 meters
F = G M₁ M₂ / D²
= (6.67 x 10⁻¹¹ N m²/kg²) (76.2 kg) (72.1 kg) / (8.54 m)²
= (6.67 x 76.2 x 72.1 x 10⁻¹¹ N m²) / (72.9316 m²)
= (36,645.1 x 10⁻¹¹) / (72.9316) Newtons
= 5.025 x 10⁻⁹ Newtons.
computer notation: 5.025E-9 Newton
(about 0.000000018 ounce)
Oh ! I just noticed the value of 'G' up at the top of your screen.
OK. We're all on the same page.