Suggestion: send this survey out to other forums, like mothering.com, but remove the giftedness requirement. Instead, insert a question asking if the respondent's child has been tested for admission to a TAG or other giftedness program. If so, was the child admitted? You could use the "No" respondents as a group of non-gifted students for comparing to the gifted ones.
I wouldn't agree with this b/c it depends on whether the OP's definition of "gifted" is the same as the schools'. In one state or district a child might be tested for TAG/GATE and not
admitted and be of much higher ability than a child who is
admitted in another state with much more liberal admission practices.
Where I live, for instance, they are of the opinion that high IQ or intelligence is not necessary to be gifted. I've been told that it is possible to have an IQ of 100 and still be gifted if you exhibit behavioral characteristics of giftedness and high achievement or high grades (you could get in with two of those three with smack dab 50th percentile ability scores if your parent was pushy enough or the GT coordinator was of that bend).
I've also been told that 99.9th percentile IQ scores don't necessarily mean gifted and had to fight for inclusion in GT for my 2e child with those IQ scores b/c her achievement and behavioral characteristics weren't consistently what they expected of a gifted student.
Unless you are going to do a study utilizing families who've had IQ testing done at some location where most of the testing is done for GT purposes, you probably are going to get some people answering who meet your definition of gifted and some who don't. Perhaps the only way to tighten the group responding is to put some definition at the beginning as to who you want responding or to ask some questions like, 'has your child had an IQ test or group ability test administered and what were the scores (subtests and composite)?"
eta: in looking at the survey entry page, I do see that there is a definition of gifted to include IQ of 125+, formal GT id in school, or exceptional ability in any one area. That is more liberally than I, personally, would define as gifted, but that is probably influenced by my experienced with over and underidentification locally to my area as enumerated above.