My understanding (do your own research to be sure)
1. You can stay out of US and enter US within 6 months (no questions asked in immigration)
2. You stay more than 6 months but less than 1 yrs (some question maybe asked depending on how many times you go in and out of country)
3. You MUST return within 1 yr. (period)
4. If you want to stay more than a yr (up to 2 yrs) - you have to apply I-131 document BEFORE you leave US.
5. You cannot stay out of US for more than 2 years.
Hope this helps.
Main purpose of GC (permanent resident) is to stay in US as resident. It doesn't work if you want to live in Nepal and have GC, doesn't make sense. You can live here for 5 yrs, get citizenship and do whatever your want. Just my opinion.