English would work for titles:

He is President.

He is Doctor. (Dr.)

"He is president" is different than "he is a president" (one among many) or "he is the president" (the one specific).

But it would have to be within the context of titles

With the specific case, the "the" could be dropped - "who is in running this rpg?" points "he is gamesmaster" which means "he is the gamesmaster".

Probably because it's a role, similar to the title?