I recently got my green card.
My Nepali passport is still valid.

Can I visit canada with green card and valid passport to meet my friends?

or
i do need to have I-551 endorsement too in my passport
http://www.can-am.gc.ca/visas/visit-visite.aspx?lang=eng


Please suggest if anyone has suggestion on it.

We drove to Toronto few months back, via Buffalo, with just the 2 documents you mentioned above (GC & Nepali passport) plus obviously a valid US Driver's license. At Canadian side besides looking at the GC and Passport, she asked us where you going, how many days, any alcoholic beverages on the car? Since we had just visited Nepal recently, our Nepali passports had the recent I-551 endorsements, I guess. But I think it will still work if the endorsement is old (like from the first time entry).

GC and passport are sufficient. People getting greencard from employment or marriage while in US do not get i-551 endorsement in their passport. The "AND" in the link you mentioned is meant to be "OR".

http://www.cbsa-asfc.gc.ca/travel-voyage/ivc-rnc-eng.html#a1a

I just took Green Card. That is enough!

muri muri dhanyabaad for the information.

Yes, GC and Nepali passport is all you need.