Hey

Every gate qualified student can apply for PSUs . 

But the shortlisting will be based on the no of  applicants applied and their marks.

The priority will definitely given to top marks.

Getting into PSUs is everyones dream, and many toppers aim that.

So PSU is a choice of toppers.

You can check PSU cutoffs of previous years to get an idea

https://engineering.careers360.com/articles/what-are-gate-scores-for-psu-recruitment