Maximum Deceleration (g) will always be equal to the coefficient of friction (μ) of the tires, IF front and rear tires have the same value for μ AND there is no down force to account for AND the suspension and road condition/surface allow for maximum traction.

In case front and rear tires have different values for μ, dynamic weight distribution under maximum deceleration and static weight distribution both determine the resulting value of μ (representing g).

In case positive down force has been taken into the equation, g will be higher than μ. Negative down force ("lift") would cause g to be lower than μ.

Notes:

For metric system users: 1 g ≈ 9.81 m/s² = 9.81 N/kg.

g-force is a term for acceleration felt as weight. It's not "just" a force, but a force per unit mass. You can read more about g-force here.

The SI unit g stands for gram. The g that indicates g-force is not an SI unit.

Most people use the capital G for g-force, however, G is the symbol for gravitational constant.

The calculated result for Maximum Deceleration is only "accurate within reason". Although a higher vehicle weight is compensated for by smaller master cylinders (so the required pedal pressure and ratio can stay the same), constructional limitations of the tires will make it harder to achieve the calculated number when vehicle weight goes up (for example, too much deformation of the side walls of the tires can cause uneven pressure of the contact patch on the road, which you can only up to a point compensate for by increasing inflation pressure).