Worry about gap seems to me to be the same old lack of understanding about why GT kids are ahead. If we parents had been spending every waking hour hothousing to get them to be ahead, then gaps would be a problem. No time for hothousing to catch them up! Yikes!
But for a GT kid who just gets stuff on his/her own, gaps get filled in immediately.
I completely agree with this -- if the gap were that big an issue, it would have been clear that a skip were unnecessary. If the child gets the basics, any gaps should be pretty easily filled. Except cursive (I do have that lingering effect from my second-grade skip -- my handwriting is horrid).
Oh, one more related thing: I'm really beginning to understand the importance of IQ testing: it's so they'll believe you when you make claims about a child's abilities, right?
This didn't help us at all because I don't believe those we were dealing with understood IQ, standard deviation, etc. We had a teacher, while looking at numbers that indicated she was unlikely to see another kid in her career like this one, tell me that 1/4 of her class of 20 was just as bright

.
This was our experience, too. It was extraordinarily frustrating. I thought testing was going to help with the school, but it didn't so much ... it more reassured me that I wasn't wrong!
Of course, this isn't the case everywhere and with all teachers, but it was for us.