But there are some PSUs which allow several other disciplines as well under advertised discipline like I found for BHEL in this link https://engineering.careers360.com/articles/bhel-recruitment-through-gate              Can you suggest me some other PSUs which might consider P&I under ME paper?

I am sorry to say that but academic qualification for BHEL recruitment is - Candidates must have passed a full-time Bachelor’s Degree in Engineering/Technology or Five year integrated Master’s degree or Dual Degree program in Engineering/Technology in Mechanical or Electrical Engineering from a recognized Indian University/ Institute. As you did your BE in P&I you are not eligible.PSUs not only take into consideration your GATE paper but also your branch in BE make sure your branch in eligible.Hope this helps. 

Please go through this link to see in which PSUs your branch is eligible.  https://engineering.careers360.com/articles/psu-recruitment-through-gate

S.NoDiscipline AdvertisedAllowed disciplinesCorresponding GATE 2018 Paper Code1.ElectricalElectrical & ElectronicsElectrical, Instrumentation & ControlHigh Voltage Engg.Power Systems & High Voltage EnggElectrical MachineElectronics & PowerPower ElectronicsPower Plant Engineering.Energy EngineeringPower EngineeringEE2.Mechanical Industrial and Production EngineeringIndustrial EngineeringMechanical Production and Tool EngineeringProduction Technology Manufacturing Engineering (NIFFT Ranchi)MechatronicsManufacturing Process and AutomationPower Plant Engineering Production Engineering  Production and Industrial EngineeringThermal Engineering  Manufacturing TechnologyPower EngineeringMEBut sir what's this about then? I just found this table ont that very page.

Thanks for the link