You must know the target's actual name to summon them via Gate (but there's no indication that Orcus's name isn't really Orcus)
As the gate spell describes:
When you cast this spell, you can speak the name of a specific creature (a pseudonym, title, or nickname doesn't work).
The specific term "true name" isn't used here - perhaps deliberately, to avoid harkening back to the (awful) truenaming magic system from earlier editions. As long as the name is a name legitimately used as a sort of proper name by the entity you're trying to summon, you get that creature.
You can't cast gate and say:
- Mr. Sucro (this is a pseudonym Orcus uses when booking hotel reservations incognito)
- The Blood Lord (this is Orcus's title, not his name)
- Orcy-Worcy (this is your affectionate nickname for Orcus)
But you can say "Orcus", and get Orcus, assuming he deigns to allow the gate to open in the first place1. There's no indication in the manuals that I can find that Orcus's proper name is anything other than just Orcus.
1He's probably cool with you saying hi if you're on an "Orcy-Worcy" basis with him.