Can I rent a bike from North Gate and return at the Southgate and get the deposit from there ?

Yes. You can return the bike and get the deposit at which section you would like to stop riding. Please remember keep the deposit receipt and take enough Chinese cash before you rent the bike. If time limits, you can go from the North Gate, cycle around the east side and then finish at the South Gate. The eastern side of the wall is more prosperous, where you can see the changes of the city and the Xi'an Railway Station. Moreover, there is also much to see near the South Gate, you may explore a bit after getting down from the wall.