The main difference is

Z is a Dirgha...when you say Z you say nepali JHEE ( when you do this your lips are positioned as "brushing your teeth" , The upper teeth and lower teeth almost touch each other and your tounge is positioned right between your lower and upper teeth but not touching them

G is a Rashwa..when you like Ja lai eekar Ji ( when you say this your lips come forward, Like a pigs lips jasto bhanchan ni nepal ma and also your tounge touhes the talu of your mouth with lips appearing as sungur ko thutuno jasto.....i hope this helps.

I had trouble saying those letters for 10 years...did not even know i had problem saying these letters until my African co worker started making fun of me cos i could not say Jay Z properly.

If you can say JAY Z correctly, you have mastered it....lol

This reminds me of a thread i created sometime ago...addressing the issue of us Nepalese not knowing how to prounce our own words. I see many nepalese not even knowing the difference between.

Hit and Heat ( Rashwa Dirgha)

Own and Won ( O and Wo )

Yes and S  (  Yes and Es)

Hut and Hot ( a and aa)