Although there are interatomic potentials to model the interaction between Ga and As, there seems to be no way to mode the cross interactions between Ge and Ga / Ge and As (unless potentials like LJ or other less accurares ones are used)
Can someone please guide me how to proceed as I am trying to model MOCVD of GaAs on Ge
The recommendation for finding potentials is always the same: search the published literature.
If you cannot find anything, you have to either learn how to create parameters for specific force fields. Again, the derivation of similar potential parameters in the published literature is a useful guideline (make sure you compare and contrast multiple such publications, since the quality of publications can vary quite a bit these days). If you are adventurous, you could also look into using machine learning to create potentials. This is in some ways more challenging but promises higher accuracy.