Girish Rajput:
I think you are talking about the Re-entry Permit. One can obtain it after GC and stay upto 2 years outside of US. But, the immigration officer can deny entry (rare cases), even if you have the re-entry permit stating that you have abandoned GC (possibly no link in US if no income, house, family, etc. in US).
As for USC, 5 yr 'continuous residence' is needed. Any outside trips should not exceed 6 months. If more than 6 months, the 'continuous residence' clock will be reset. So, technically, one can get USC by being 2 1/2 yrs in US during a 5 yr period by being 6 months in US and 6 months overseas alternately. I do not see that working in reality.
There are exceptions if your job sent you outside, but you might have to prove that you had residence/family, additional links (finance), etc. in US during your outside presence. Those are easier said than done and who wants to be away from their family for such long periods of time.