Do I have to prepare for GMAT Or IELTS is enough?

So you need to understand that the objective of GMAT and IELTS is completely different. While both are entrance exams to study abroad, GMAT tests your academic proficiency and IELTS, on the other hand, tests your language proficiency. 

So GMAT and IELTS both are equally important for your profile. Another important thing, since you didn’t mention about the course that you wish to pursue, but it’s important as different exams cater to different courses. For example, GRE is the entrance test that is given for MS courses and GMAT is given for business courses i.e MBA and MiM. 

Hope this clears your confusion.