You switched one param as mean is the second one, but correct :P... so gRandom(10, 50, 0, 100). This will result in the heigest chance to roll between 40 and 60. 99.7% of the values will lie between 20 and 80.
It might be easier to use gRandomBetween though, this automatically calculates the mean and deviation so that the above example would be equivalent to gRandomBetween(20, 80) with the difference of having a cap between 20, 80 instead of 0, 100