In addition to being an SAT and ACT tutor, I am also a GMAT tutor and a GRE tutor. The SAT, ACT, GMAT, and GRE are actually very similar, much more alike than most people realize. The content that underlies the tests is almost exactly the same, especially with regard to the SAT, GMAT, and GRE (the ACT differs a bit more from the other three). And all of the tests are essentially reasoning tests so the kinds of critical thinking and problem solving skills that they test are also essentially the same.
The GMAT and the GRE are more difficult than the SAT or ACT, but again the content largely overlaps (there are a few concepts on the GRE and GMAT that go beyond what is on the SAT and ACT, but then again there are some things on the ACT that go beyond what is on the GMAT and GRE). What sets the GMAT and GRE apart is that the questions require more creative problem solving and more rigorous critical thinking.
But that is sometimes just what is needed for people studying to take the SAT or ACT. GMAT and GRE questions often provide an opportunity to push students to a higher level of reasoning so that they can see just what type of critical thinking and problem solving skills need to be applied on the hardest SAT and ACT questions. Some questions are just too hard or require knowledge or skills that go beyond what is on the SAT and ACT, but many other GMAT and GRE questions could just as easily be SAT or ACT questions (and vice versa actually…in fact on the June ACT there was a Math question that I would argue would be on the very top of the scale of what could appear on the GMAT or GRE).
For example, SAT Sentence Improvement (SI) questions are virtually identical to GMAT Sentence Correction (SC) questions except that the latter version is significantly harder. Nevertheless, when I am trying to teach an SAT student how to approach difficult SI questions I often use some really good, level-appropriate GMAT SC questions. The greater focus on logic and meaning, the kind of trickery, the need to use the wrong answers are all things that are typical of GMAT SC questions and they are applicable to hard SAT SI questions as well.
And often when I am trying to teach students how to draw proper inferences I like to use GMAT Critical Reasoning inference questions to make the point. The hallmark of drawing proper inferences is understanding that you cannot deviate too far from the evidence provided – the inference needs to be a conclusion that can be drawn with near certainty, not a conclusion that might be true but is not necessarily inferable based on the information provided. That is a key skill on SAT Critical Reading questions but it is a point that is difficult to illustrate in isolation on SAT questions. So I often use GMAT CR inference questions with my SAT students to make the point and then once understand the concept they start to see how to draw proper inferences on the SAT and ACT.
Using GMAT and GRE questions en masse without understanding which ones are applicable to the SAT or ACT is probably not a great idea. In general the Math questions translate well, but many are often much more difficult that those that appear on the SAT/ACT. Nevertheless, select GMAT and GRE questions often supplement SAT and ACT questions very well and give students the kind of training that they need to handle the most difficult questions that they are likely to see on the SAT and ACT.