There are a couple of places you can get authentic Greek eats in the Golden Triangle DC. You probably already know (and have stood in line for 20 minutes) about the one on 19th between M and L, but you may not know about the other authentic Greek place a few blocks over: Port of Piraeus at 1125 21st Street NW.
Eating here reminds me of when I was at the real Port of Piraeus in Greece. It's that good. The menu offers traditional sandwiches, salads, soups, entrees, and desserts. The gyros are excellent! And they offer daily specials.
This place is usually busy but the ordering line goes pretty quickly--and definitely worth it. There's seating inside and outside. They serve breakfast, lunch, and dinner (closes at 6) so grab dinner on your way home by ordering ahead online!