Cala Moresca Restaurant, located poolside at Cala Luna Boutique Hotel is one of the few places in the Tamarindo area with a gluten free menu (it’s part of the standard menu, with gluten free items marked with a GF icon). There is a pretty good selection of gluten free items on the menu and the staff are reasonably well aware of gluten free needs.
The daily buffet breakfast is also a great stop for people looking for gluten free items. There are all the breakfast standards like bacon, fruit, cheeses, as well as a selection of lunch dishes like rice and vegetables. They also offer some pretty fantastic gluten free pancakes and you can get corn tortillas instead of bread, you just have to ask.
Cala Luna is the only place in the area that I have seen gluten free pancakes and they are worth the trip if you have a craving!