First of all please visit http://gate.iitm.ac.in/. Once in the page, click on 'click here to apply online'. you will be taken to the page GATE Online Application Processing System (GOAPS), Here there will be an option "Forgot enrollment ID or Password", you need to click on the tab. Next you will be taken to a page "Forgot Login credentials". Next you need to click on the tab 'Request for an OTP' to get One Time Password via SMS/Email.
Hope this was helpful.
Wishing you al the Best
Incase you forgotten password you can retrieve it via login page where you can click on 'Forgot Password' link.
Afterehich you shall receive link to update your new password on your registered email ID.
You can edit and renew your password and login again to get all details.