There is no such percentage criteria if you want to go for M.Tech through Gate but yes, if your aim is to get recruited in PSUs then some would be asking for 60% whereas PSUs like CEL would ask for 65% so basically it varies PSU to PSU. To check the complete eligibility criteria of Gate, you can go through the below article -

Gate 2018 Eligibility

Also, to know al about PSU through Gate 2018 you can consider below link -

PSU through Gate 2018

Hope this helps!!!