If you assume that "Earth" is part of the D&D multiverse (the "Great Wheel" cosmology), then the standard D&D Orcus probably knows of it and involves himself in its cults, as does the standard D&D archdevil Dispater and other demon princes and archfiends. If he's not the same entity as the Etruscan deity Orcus then he probably, at least, poses as him in an attempt to poach his worshipers.
If you assume that the Olympian Twelve rule the cosmos and are known by different names in different cultures, then Orcus is surely just the name the Etruscans knew Pluto/Hades by and can be considered to be essentially the same deity. This may be true even if other pantheons exist on your fantasy version of Earth. For example, perhaps the Norse/Germanic deities are completely separate from the Olympian pantheon, but the Etruscan deities are simply the Roman deities under different names. The demon prince Orcus may still attempt to poach Pluto's worshipers.
Big Mac wrote:If Orcus can be a deity in the Ghostwalk setting, could he also be a deity in the Gothic Earth/HR5, but a demon in other parts of the D&D multiverse?
The simple answer is yes. A god may have a different divine rank on different worlds (for example, Tyr is considered to be a greater deity on Toril but an intermediate deity on most other worlds) and a god might even be dead to one world and alive to another (Zinzerena is dead on Toril but alive on Oerth, for example).
Note that Orcus was considered to be both a deity and a demon in 2nd edition; the two categories aren't mutually exclusive. Lolth is both.