I didn't read all the replies, I'm late to this topic.
In order to get into GATE there are 3 options. 1) Score above 95% on the OLSAT. Optional, a student can take it once a year starting in 3rd grade, outside of the school day but district administers. 2) Score above a certain level on both math & english versions of the state testing. 3) Get a private report from a list of approved psychologist. Parent pays the cost of the evaluation and I'm not 100% sure of what test would be administered.
GATE doesn't start till 4th grade, before that advance work is by discretion of the teacher/principal. Once you are 'labeled' gifted you keep the label but it doesn't affect anything after 8th grade. What GATE involves varies from school to school, and sometimes differs by teacher. In junior high it guarantees admission to the honors humanities track. Advanced junior high math, is by a math 'pre-algebra' test administered at the end of 6th or 7th grade and one of the only ways to get into the honors high school math.
My district also has a dedicated classes for the gifted/highly motivated 4-6th graders. Enrollment is based on primary on a 3rd grade teacher recommendation & various test scores/grades, supposedly not OLSAT but they do see the score. GATE identification not necessary but few kids get into it who aren't already identified. Competition is stiff and done behind closed doors.