GT Certification is different from the 30 hours required by the state.
In Texas, gifted/talented certification is not required in order to be qualified to teach in a gifted program. If you want to get the certification on your teaching credentials, though, you can certainly do that. It involves taking and passing the TExES GT Supplemental 162 exam. Once you pass it, there will be an additional charge to have it put on your teaching certificate.
You’ll notice it is one of the limited administration CAT tests—which means it is not given as frequently as other certification exams and it is available only by computer. There are several testing centers in this area, and it’s important to register early since spots fill up pretty quickly.