The special programs start in first grade with the Takoma School -
http://www.montgomeryschoolsmd.org/curriculum/specialprograms/elementary/primary-magnet.aspx. With respect to Kindergarten, it depends on the school and the teacher. Our school had reading and math groups so my DS was with kids at his ability level. He is generally 2 grade levels above the curriculum but he is not the only one so he has a peer group and they push each other. I do supplement at home but that is because I don't think the math is challenging so after my DS takes 5 minutes on his math homework, I sometimes him one more sheet that challenges him. We also make him practice writing (again, nothing crazy - it takes him a few minutes to complete if he focuses - an hour if he decides to decorate the page with pictures in an effort to wait me out). Not every day and I think it helps when me and my husband are there to explain concepts.
HGCs start in 4th grade and testing begins before that, along with teacher recommendations, etc. Can't speak to what it takes to get in. Have seen posts where kids are reading at high school/college level and other posts where kids are just a few grades above curriculum. As long as your child is enjoying school and continuously learning, then he or she is in the right place.