One day I casually visited Grill Square in the Omonia Square.
I don't know the exact address, but, as you see in the photo, it is directly facing to the Omonoia Square and just next to Macdonald's.
They allow both take-out and eat-in (self-service) and the price does not differ. Besides gyros and kalamaki, they sell also grilled chicken.
I had a pork souvlaki in pita (1.80 euro) and a bottle of water.
Fine. No complaint.
The best thing about this place is the seating space. In Omonia Square there is the famous Gonia souvlaki shop, but it does not have seat. I have to stress also that the shop is clean and bright enough that even a lone woman can eat-in without feeling uneasy. I will be probably back.