My personal opinion is that trying to do "test prep" for G/T testing is unnecessary, and puts unnecessary stress into parent's lives, as well as that of the young students. Most children do well if they've had a hearty breakfast and a good night's sleep before school testing is done. If kids have grown up in an enriched environment, where they've been exposed to books and experienced a variety of family activities (ie, trips to the park or zoo, helping with cooking, gardening, etc) they will likely do very well on the tests.
For example, what I mean is, it's far easier for a child to understand 1/2 + 1/2 = 1 whole, if they've cut a pizza, or used measuring cups before. Or if they're presented with a picture of a kitten, and a baby, and then a picture of a cat, and an adult person, can they make the association of young versus old? Probably. I wouldn't go out and buy flashcards to "cram" for any test of this nature, but just see how the child does on their own. (If a child's test performance is "artifically inflated," but they're going to struggle to keep up after getting into a G/T program, is that really what a parent wants?)