hi,

some of the psus which hires mechanical engineer is ongc, iocl, bhel, mdl, npcil, hpcl etc. you may also apply in research institute like  barc, drdo etc.

year to year number of vacancies  varies and cutoff score depends on the question difficulty level, number of participants etc.

if you have 70+ marks in total of 100 in gate exam, you have decent chances to get calls from psus.