The actor Eli Gelb has really found a niche for himself as "unassuming guy in a three-hour play who's able to avoid getting directly hit by the drama." Before catching him in Birthright, the new off-Broadway play from Jonathan Spector, I'd last seen Gelb as the sound engineer Grover in the Tony-winning Stereophonic. In that play, which was clearly inspired by the making of Fleetwood Mac's Rumours, he begins the job ambitious and a little naive b…