Hi,

You can apply through obc b category for gate 2021 exam. But a creamy layer obc you will not get the benefit of the stipend if he applied through the obc category in gate exam. Read the general instruction given by aicte to student

https://www.aicte-india.org/schemes/students-development-schemes/PG-Scholarship-Scheme/General-instruction

The only benefit you will get to apply through obc category is that of cutoff.  Minimum qualifying marks required to qualify the exam will be considered from obc category.

I will suggest you to apply through open category if you want to avail the benefit of stipend.