Hello dear, 

This is individual decision of specific PSU whether they want to hire candidates solely on the basis of GATE score or conduct additional tests. There are some PSUs that use GATE  scores only to screen candidates and then conduct GDs and PI to pick the best candidate. There are some who use GATE scores directly for recruitment. 

For more information, please visit the link below: 

Https://engineering.careers360.com/articles/psu-recruitment-through-gate

All the best!