Hello! If you have been able to gain accommodations for other exams, then you have a chance to be able to do the same for the GMAT. (It isn't the case that it's harder to gain accommodations for the GMAT than for, say, the LSAT.) I can't guarantee anything of course—but that's good news.
It is the case that they ask for documentation from the previous 3 years unless the issue is something permanent (such as blindness), since many conditions can change over time. So if it has been 4 years, you may have to repeat the tests / redo the documentation.
I wrote a (long!) blog series on this about 5 years ago—here's the link:
https://www.manhattanprep.com/gmat/blog ... at-part-1/The big picture information regarding how the process works is still accurate, though some of the specific details may have changed over time. In that article, I interview Dr. Tova Elberg, a clinical psychologist who works in this area and has helped many students with the necessary testing and their applications for accommodations; I highly recommend her. She does still work in this field, so you may want to contact her for help. (Of course it's smart to ask 2 or 3 people, compare prices, etc, for this kind of thing—the testing is expensive! Ask whether they have worked specifically on applications for the GMAT, as there can be differences from exam to exam. Ideally, you want someone who knows the ins and outs of the particular exam for which you're applying.)
I will note two things for everyone reading this:
First, it is not the case that anyone can "ensure" they will get accommodations by paying someone to conduct tests or work on an application for accommodations. Doctors who perform this work will conduct the necessary tests and then will only help the person to apply for accommodations if the tests show that the person is indeed eligible based on the specific rules and requirements for the exam in question.
Second, if someone does gain accommodations, it is not the case that this person is being given an advantage on the exam. Rather, the testing demonstrates that the person is at an objective disadvantage under the standard testing conditions; in this case, specific accommodations are granted to allow the person to take the exam under conditions that would normalize their testing experience, given their particular issue or issues. In other words, the accommodations are given to "level the playing field"—to make the test fair for everyone who takes it.