The alternative would be to do outside supplementation and lots of extra curriculars. That's what my parents did when I as a child and there was no such thing as G&T programs (at least here I lived). If you otherwise like the public school and the child wants to stay with his/her friends, this could work and would probably be cheaper than private.
As a practical matter, I think this is really difficult to do. I had this problem with my 1st-grader last year; he was bored to death at school. But my kids get off the bus (public school) at 4:20pm, and after a full day in school, my son really needed to run around and be outside playing. He had sports practices many nights, and we like to eat dinner together as a family most nights. So I decided I wasn't taking him to Score or some such "academic" extracurricular. The school should have been meeting his needs. But to find academic challenges outside of school almost would have seemed like a punishment for him.
8:25, I am disheartened that your kids did not get in with those WISC scores! My dd had a full-scale WISC score of 130 and she DID get in (as part of the screening pool, so we did not self-refer or appeal). I will be referring my son, though, so that information is scaring me, as I don't think pulling him out of our public school is an option for us. He would be devastated if we moved him.