Not sure how many will find this helpful but I recently have been craving gyros on a daily basis and have tried a few of the gyros in the South End. Here are my thoughts.
Giorgiana's: Right across from the BCAE on Tremont. Great pita, super hot, sliced lamb meat (not the processed stuff), but very greasy, and only comes with onions and tzatziki unless you ask for the other stuff (I like it with lettuce and tomato, too). Overall impression is that is has the gamiest lamb flavor of all, but is a major gut bomb, and very, very greasy.
Rome Pizza: This place is up on the north side of Tremont. More of a pita wrap than a gyro, but very clean and easy to eat, with a compact, simple flavor. Meat was the pre-processed stuff but sliced thick, and there was the right amount of tzatziki. Biggest downside was the thin, cold pita bread. I'm sure they'd put it on the griddle for a minute if you asked them to. That might help it puff up nicely. Overall, could be better, especially with hot bread. They are small enough that if you're hungry you could eat to.
Billy's Sub Shop: Up on Berkeley St. on the way to the Back Bay. Nice greasy spoon that closes at 4 PM on weekdays. Very hot, fluffy, yeasty pita, pre-processed gyro meat but thick-cut, well-charred and tasty, with fresh veggies. Holds well together, but the main problem was there was just too much tzatziki, and it was really thick and tangy. Felt more like it was covered a goat cheese spread than a yogurt sauce. I'd still get it again. Their burger, unfortunately, tastes like a highschool or hospital cafeteria...
Harry O's: This is the little pizza slice joint next to Morse Fish on Washington St. where all the kids from the highschool across the street hang out after school. The bread here is OK. Meat is OK, somehow not salty enough. Sauce is OK. Overall, it's OK. Main gripe was the onions were cut too thick and had too much of a sharp onion flavor on account of it. Sounds super picky but what can I say, it kinda ruined it. I'll eat it when I'm super hungry for gyros and it's the closest one to walk to in bad weather.
Overall, if Rome Pizza put more meat and had a better pita, I think I'd like that one the best so far, followed by Billy's if they went easy on the sauce.
Some that I haven't tried:
1. South End Pita
2. South End Pizza and Grill (do they even have gyros?)
3. New York Pizzeria (same question)
Anyone who knows about these other ones for comparison's sake, please chime in. I'll eventually hit the rest and post about those as well. FYI I'm not looking for the holy grail of gyros and I don't care if they're not super "authentic." I love even bad gyros. I loved them since elementary school when they came out on a foil tray. I just love them, love them, love them. And, I am feeling lazy so I do not want to leave the South End. I know about Zo's and that crazy delicious lamb thing that StriperGuy posted about a week ago, and also the places in the suburbs.