How to introduce bias distribution for angular distribution in GPS ? and how to find RNDM?

Hi Experts,
can you please tell me how can i introduce biasing in GPS ?

from the manual, i came to know about this, but i am unable to understand this RNDM and how it is calculating theta ?

can anyone please solve my doubt. it will be very helpful.

“RNDM” here is a placeholder symbol. It represents a random number thrown from a flat distribution in [0,1). In Geant4, this is implemented via the function call G4UniformRand().

But i am using GPS not particle gun then how can i implement this in my GPS code ?
and between 0 to 1, how can i know this random value i should choose ?