Give Liamos Market a call. 603-882-1381, 295 Lake St Nashua not sure if they do gyros, but maybe they can guide you.
Here's a link that may be helpfull. http://www.hippopress.com/food/food05728.html
I have never had a gyro so can't help too much, but when I worked in the area years ago, I do remember people liking Christopher's for gyros.
The area has a few decent restaurants, but certainly you aren't going to find what you have been used to without a bit of a drive.
I've been in the area for almost 30 years and things have gotten better, but you have to really search out the good places. Chains are just taking over.
Main St in Nashua has a few good places. Non of them are Greek.
http://www.thefrenchbistro.com/ nice little place in Milford,NH
Now what you will not find is great pizza, sorry to say. Not even good. I have just started making my own.
Although I have read that Espresso NY style pizza on Main St in Nashua is good. Just haven't made it there yet, and also maybe Amato's on 101A in Amherst.