CAT

Hi Nikhil,
CAT is the short form of "Common Admission Test" and is conducted by IIMs to select the merited candidates for admission in various management courses of the Indian Institutes of Management in India. However, CAT has now become the entrance examination for several other leading institutes apart from IIMs.
On the other hand, GMAT is known as the "Graduate Management Admission Test" and is a computer adaptive standardized test in mathematics and the English language for measuring aptitude to succeed academically in graduate business studies throughout thousands of Business Schools/Universities world over. International Business schools commonly use the test as one of many selection criteria for admission into an MBA program. It is given at various locations around the world.
There is no set strategy for these two examinations. Please follow the link mentioned below for preparation strategies:
https://www.shiksha.com/mba/exams/cat/preparation-tips
https://studyabroad.shiksha.com/exams/gmat/when-to-start-preparing