There is no restriction on the number of GATE attempts on the basis of caste and age. An aspirant belonging to any category, be it SC, ST, OBC or General can enroll and appear as many times as they desire. There are even many lecturers/professors who appear in GATE to keep their knowledge/skills updated.Also for SC category most of the psu allow students to apply till 37 years or 40 years of age.

Hope this helps.