Lobsang Tsering explained me about the etymology of Buddhist cast names.
Tamang comes from རྟ་དམག tamag, horse warriors, when later on its pronunciation changed to tamang instead of tamag.
Gurung originates from སྒོ་སྲུང་། gosung, gate keepers or gate protectors, they originally were hired to do such tasks. Also pronounced སྒོ་ཧྲུང་བ།.
He also pointed me how the demonym particles are a bit different across countries, they can be པོ་ པ་ བ་:
- བལ་པོ། Nepali person
- བོད་པ། Tibetan person
- རྒྱ་གར་བ། Indian person