As you have asked about the official website of the GMAT exam for MBA course, I am giving you information about it, check below for the details

Website:
mba.com/global




On its homepage, you will find many sections.

It has following menus:

The GMAT Exam
Find & Compare Schools
Plan For Business School
Buy Study Materials

It also has following:
Register for the GMAT Exam
Free GMATPrep Software
Let Schools Find You
Diversity in the MBA