Of the items you list, garlic will probably be the hardest to avoid. It is used a lot in Spanish cooking, even in things that don't seem to have a garlic flavor.
Most of Europe is very sensitive to celiac disease and so avoiding or limiting gluten will not be too hard (though you will miss out on a lot). Many tapas have a crostini base, but if you are only sensitive and not allergic, you could just skip eating the bread. But many tapas are also either fried or have garlic so it will limit your ability to eat tapas. The good thing though, is that you can easily pick and choose and usually can see them on display, so you will better know what you are getting before ordering.
Restaurant menus will be pretty diverse and most will have something you can eat. Seafood will be very common and as long as you avoid a garlicky butter sauce, you should be fine with that. Fried foods are not nearly as common in standard restaurant menus as they are in the US.
I would also bring a list of things you need to avoid written in Spanish to show the wait staff. English isn't as well spoken throughout Spain as it is other places in Europe and having it written down avoids confusion. People are very nice and will politeness on your part, will work to accommodate you.
Menus tend to be posted outside, just like France.
