As PSUs accept Gate score so it would be better to work really hard to crack this exam with good score and to check the complete preparation tips, important topics and books which you can follow, kindly go through the below link-

https://engineering.careers360.com/articles/gate-how-prepare-for-mechanical-engineering-me

And once you qualify for the PSUs then interview tips are given below -

How to face PSUs interview?

Good Luck!!!