There are some parts of Garland that are actually in RISD, so have you checked to be sure of what school district the house is in? Unfortunately, there are many people that give false addresses to be in a school district. I know of many that do it in GISD. It is illegal and I personally would hate to live a lie. It would seem to me that it would be a bad example for your kids.
Yes, GISD is a very good school district and with the Choice of Schools, you are not bound to the school that is nearest to you. There is an excellent magnet program for Talented and Gifted kids and all of the magnets are exceptional schools. Your children can qualify for academics, art or music. There are three Academies for elementary kids that are academically gifted and they are all excellent - Hillside Academy, Kimberlin Academy and Walnut Glen Academy. For the Academies, your child must test in the top 10%. There are also two Math and Science magnets - Beaver and Watson and the Classical Center at Vial. For the magnets, your child must test in the top 25%.
There are many other good elementary schools in GISD. Most of the elementary schools in North Garland - Abbet, Armstrong, Cooper, Sachse and Rowlett are good as well as many in other areas of Garland. I am not familiar with Hickman Elementary, looks like they are rated Recognized. I would suggest you visit the campus and see what you think. I have heard good things about Routh Roach also.
Moving in the summer, you will miss the deadlines for Choice of School, but I believe they make exceptions for students moving into the district. I would contact the district to find out what your options are. GISD has many good options!