I do always have a health insurance while I am here in USA.
I am GC holder
I would be visiting Nepal for 3-4 months.
During that period, do I need to have a health insurance(Obamacare stuff)?

You don't need to have health insurance while you are in Nepal. Even the health insurance you have now most probably won't cover anything while you are there.

However, if you do lose coverage for this 3-4 months, what are you going to do after you come back? You won't have any coverage then! You will have to wait until the open enrollment--which is in 2016 to get the coverage again.

However, if you do lose coverage for this 3-4 months, what are you going to do after you come back?
special enrollment, that is another part of Obamacare fun :)
Ram has a health insurance from companyA.
Ram lost his job so does the health insurance with companyA
Ram finds a new job and the open enrollment date is way far.
In such scenario no matter Ram finds a new job with companyB in the same city,state or different state(unless the state has specific different rule), Ram can get health insurance using special enrollment.