Yes it is necessary if you intend to apply to that PSU and they are accepting applications prior to GATE score.PSUs have their own defined time period during which they accept applications from GATE aspirants. Some accept forms before GATE some after.For example, IOCL, Power Grid, NTPC generally accept applications during month of January, before GATE exam while ONGC and some others accept after GATE.The point is, if you want to be accepted in a PSU, first thing you should have applied to their respective recruitment process.Yes, they need your Gate score card, and in case you filled their form by spending money but later didn't qualify their cutoff, it would seem like a waste.But if you did get the required score, and you haven't filled the form, they won't accept your case as their application acceptance process is over. This is mentioned in all PSUs advertisement.So the advice will be, fill the form when they are available if you want to apply to that PSU, so that you don't regret later. As many students forget to fill, or are unaware of this before GATE result but then they don't have any option to apply.Hope this helps.