It is common to prep kids for this test in NYC. The programs are not geared for genius children, just brighter than average. Just like a genius doesn't need to prep for the SAT"s but a bright kid can get some help from it, same with the G&T test. Getting your child into a G&T elementary helps to get them into a better HS, which helps to get them into a better college, etc.. Some people look long term and others don't care. It may or may not help every kid, but it doesn't hurt and it may help.
OP - look at the stats for which schools/programs feed into Stuyvesant, Bronx Science and Brooklyn Tech. If your local elementary/jhs has decent numbers I wouldn't worry about it. If they feed into a local hs with high drop rates, high pregnancy rates, etc., then I would look more seriously into the G&T elementary program. If you have a good local high school it's not as big of a deal.
Remember that there tends to be more well to do kids in the G&T programs, that can donate more to their schools and help out when there are budget cuts. The parents tend to be better educated and more focused on education for their child. It will make a difference in the overall class experience and the friends that your child will make. It's not to say that you can't have great kids in your local school, but it will be a bigger mix and include more kids that have parents that are less involved, focused on education, etc..
Just something to think about.