My youngest (DSnow 6) skipped kindergarten. He had been to private pre-school where they did most of the things that are done in K the year before. Entering 1st grade just before he turned 5 was perfect. He made lots of friends and really groved on the routine. After half a year he was ready to move on.
For HG kids, single grade skip may not the end-all answer. The pace may be too slow--it'd be great to have compacted grades (half yr 1st, half yr 2nd, half yr 3rd, etc.)
Luckily we have a PEGs class and he went into that for the 2nd half of the year and the rest of elementary school. They can continue to subject accelerate in that setting (4th grade spelling, 7th grade reading, etc.) He can now do long division and manipulate fractions without having to sit through more addition/subtraction.