You can hand that GELU definition to a mathematician and they can interpret it as a function of a real number b. The definition does not depend upon b being a floating-point number with a particular bit representation.
In contrast, the fast inverse square root really exploits the bit representation of a floating point input to cheaply compute an initial guess.
In contrast, the fast inverse square root really exploits the bit representation of a floating point input to cheaply compute an initial guess.