Staying at the gunter, and if your primarily looking for just Tex-mex on the riverwalk, then old army gave you the best advice, mexican manhattan and casa rio. I would probably add rio rio cantina to that list as well. If you want to experience a broader selection of mexican fare, try the acenar which is right down the street from your location. Off the riverwalk, rosario's is the best, just south of downtown, la gloria is at the pearl brewery on the north end of the riverwalk-you can reach it by cab or on the river barge. The food is not your typical tex-mex food,but is very good. Check their online menus.