As per GAMCA guidelines,when they perform medical examination, heart beatings must be regular and consistent and heart function should be normal and free from congenital defects and organic diseases. Now, if your wife had a surgery 20 yrs back, it would definitely had done for some kind of congenital issues. The point here is if she is absolutely normal and the problem is completely cured, there is a chance for hope as this problem is a non-communicalble one. The another reason why GAMCA is very strict about the medical tests is, the employer has to bear all the medical related expenditure once they issue the visa to your family.
Did you do her medical by giving her passoport details and did they give you an official document that says she is UNFIT? If the details were not entered in GAMCA database, you can go another GAMCA center and ask for a general medical check-up and discuss with that doctor. If they can manage a FIT certificate, then back in Oman, they usually do only blood examination. But, it varies heavily on the center and from which country you are coming from.