Hii Narasimha..

Basically GATE and PSU (Public Sector Undertaking) are two completely different terms and have a strong link between them.

GATE is an aptitude test in engineering, as its name suggests, and qualifying this opens up opportunities, like MTech or PhD and also many PSU jobs.

PSU are government owned corporations, where the central government hold the major share (more than 51%). PSUs offer large category of jobs starting from engineers to clerks and assistants. The engineers recruitment for PSUs is through GATE exam, means if you qualify GATE, you can apply for those posts. Some major PSUs include NTPC, BHEL, CEL, BPCL etc.

Hope this helps you. All the best.