Both of my sons went through the GT program in Garland. One will be a senior and one will be in 8th grade. It is an excellent program and I would recommend it to anyone. My understanding is that when they test, they accept the top 10% into the program. It is not a lottery. They may not have spots for all of the kids that qualify, so they would be put on a waiting list in order of their scores. I believe the principal has some leeway on the selection process. For instance, if they have 20 kids that test >95% and five more that test at 90%, they may choose not to accept the lower scoring kids and save the spots for higher scoring kids the next year. If your child tests in the top 5% for K, then they should be able to get a spot. Each of the three GT elementary schools only has one GT K, so that means there are 22 spots for each school. In 1st grade, they add a second GT class per school, which means there should be 44 total spots for 1st grade per school. I believe they add another GT class in 2nd grade for bilingual kids (but that may even be in 1st grade now.) So, it seems to me that your chance of getting in for 1st grade would be about the same as getting in for K. I am surprised that especially the >99% classmate did not get a spot for 1st grade.
Once a child has a spot in the program, then they are guaranteed a spot every year, unless their grades, test scores or behavior causes them to be exited from the program. So that does make it harder for kids that try to enter the program later, because the spots are likely to be filled already. In 5th grade, they have more spots open up, because the ratio of teacher to student is raised.
There is a second magnet program that is based on a lottery. There are two Math & Science Technology centers and one Classical center. They only have to score in the top 25% to qualify for those spots and then have to be selected in the lottery.
It sounds to me like the two older classmates should have qualified for the GT program, but may have been put on a waiting list for a spot to open up for them. They could have also entered the lottery for the other magnet schools and not gotten chosen. If they are on a waiting list, there is still a chance they may get a spot even after school starts in the Fall. Sadly, many people don't notify the school district of their plans and so they often have kids that just never show up and after the first week, they give those spots to the kids on the waiting list.
The school district is required to provide GT enrichment at all of the schools, not just the magnets. At the magnet schools it is a full day/everyday program, but at the other schools it may be only a pull-out class once a week. Not all GT kids choose to go to the academies. There are several other really good elementary schools in Garland ISD and I don't think a gifted kid would be bored in them. It really depends on the teacher and the school, as to how well they provide stimulation for gifted kids. And parental involvement is essential to be sure that your student's needs are being met. I think it is hard for K-2nd grade, because the kids are at so many different levels based on what parents have helped them with and what (if any) preschool they have had. Regardless of how smart a kid is, they are not all at the same level in K. My older son began reading at age 4, but my younger son, didn't start reading until K. From my experience, most of the GT kids are reading by the middle of K.
If you want your child to go to a GISD magnet, then I would definitely have them tested and enter in K. If your daughter just turned four, then she should be able to test this fall for K the following year. I do know many kids that were not in the GT program in K and came into it at another grade level, so spots must open up each year.
