If they are testing her for giftedness, then it is not something that can be studied or prepared for in the way you would ordinarily prepare for a test. It is not going to be a test of her knowledge, per se. It is more of her thinking processes. My daughter was in private school for several years, and this year she entered public. (3rd grade). I asked them to test her (and actually was very frustrated at how long it took them to do so). I also was in a gifted program (starting in middle school) and was tested. What I recall, and what my daughter remembered is questions like...
Football is to soccer as tennis is to: a) basketball b) hockey c) track d) baseball. That sort of thing... or figuring out patterns in a series of numbers. Or perspective things, like if a bird flies across the yard... which picture is what he will see (then with several pictures)... that sort of thing. It is NOT going to be something that you can teach her.
The only thing you can do in advance is to not make a huge deal about it, but let her know that the teacher is going to do this testing game with her. She is to do her best, but there is no grade. It is more to find out how she thinks, not passing/failing or graded. Then make sure she eats well and rests well the night before and the morning of the testing. Let her know that if she doesn't understand something she is allowed to say so, and there will be stuff she doesn't know... so to expect that, just to make her best guess.
Good luck.
P.S.
Everywhere treats "gifted" differently. Where my daughter is, they have a one day all day pull-out program, where one day a week she goes to a completely different class for the entire day with a group of gifted students like herself. Hers happens to be at an entirely different school than her "regular" school. So if/when her results come back and you are deciding whether to allow her to participate, ask questions about what the 'program' does/offers. It may or may not be a good fit for your daughter. Some schools, they are out for an hour each day or some other regimen. The poster who said it is important for YOU to know if she qualifies is correct... whether or not she does anything with the school as far as giftedness programs. You will need to be supplementing her learning experience at home if she is gifted. If she is gifted, you will likely find differences in your daughter and friends' kids... for ex.: my daughter will FIND something to learn if I don't provide it... she LOVES doing those "summer bridge" workbooks... all year round. She gets out my son's old text books and reads them for fun (he is 3 yrs older). She learned to read at 3.5 yrs and was a competent reader by her 4th birthday-- because she begged me to teach her starting at 3. Almost anything she learns for the first time, she "picks it up/gets it" after the first time/example... sometimes it takes two... but with most kids it is 3,4, 5 times... she is bored by then and ready to move on. She started "complaining" early (like kindergarten) that things at school were too easy. Not just the reading (the other kids were still learning but she was already a fully independent reader reading at a 3rd grade level) but the math also. She complained all through private school years that math was boring (she loves math.. but it was not challenging her at the rate they were teaching it). That's the biggest thing with gifted kids... they learn new things QUICKLY .. so when the teacher is going through a new math concept for the 10th time (most kids need this sort of repetition)..the gifted student is sitting there wanting to scream "I GOT IT ALREADY!" and becomes bored or frustrated. It's not that they are smarter or know more... but they learn differently and with less repetition almost ALWAYS.
Good luck.
P.P.S.
Just because she tests gifted does NOT mean that they will skip her a grade. Gifted does not mean that you move ahead in grades. It means that you benefit from different or more in depth teaching methods. The only day she comes home from school and tells me anything about school, is the day she goes to Challenge class. They write poems, do puzzles, problem-solve, play board games during any free time, learn an amazing amount of detail on a single subject (a weird bird from Australia or cactuses in the Mohave or whatever)... she gets SO much more out of that one school day. And her "regular" teacher doesn't require her to make up what she misses in class that day. She does FINE on all her tests even without that day in class. She still makes 96 -100 in EVERY subject on her report cards.