Hi. Correct me if I'm wrong, but isn't GATE = Gifted and Talented Education? I'm not sure what you mean "completing GATE at early stage helps" ~ the classes are attended throughout one's schooling....
If you're talking about getting accepted into a GATE program to begin with, different school districts across the U.S. have different ways of identifying which children qualify...In my area, they take the top percentage of performers on statewide achievement tests, although I've heard of other areas where they screen all students with IQ tests, etc. There really isn't "preparation" for those tests in an academics sense, I'd just ensure your child was well rested and eats a good breakfast the day of the testing. If she happens to be sick, you might ask if the school would let her take it at a different time.....