The m is for meters. The s is for seconds. Its a universal physics formula.
The 100 come is because the (1inch)2.54 is in centimeters, so multiply by 100 to get back to meters.

So if the universal formula for earth-like gravity is
G = (-9.81m/s2)
So if we are using 1quant = 2.54 cm, then 1meter = (100cm/2.54quants) = (100/2.54) = 39.4
So to convert our Universal formula(which uses meters) to 3DGS our formula(which uses quants)
G = ( -9.81m / s2)
== G = ( -9.81m ) / s2
== G = ( -9.81m ) / s2 * (quants/meter)
== G = ( -9.81m * (100cm/2.54quants) ) / s2
== G = ( -9.81m * (100 / 2.54) ) / s2
== G = ( -9.81m * 100 / 2.54 ) / s2
== -386 /s2

So in our case, assuming our scale is 1quant = 10cm, our formula boils down to
G = ( -9.81m * (100 / 10) ) / s2
== G = ( -9.81m * 10 ) / s2
== -98.1 /s2
So we should use ph_setgravity(vector(0,0,-98.1)); in order to match our scaling.