Dear aspirant

There are many free sources available for gate mock test such as-

Mock tests- Aglasem

Made easy gate- Free demo test series

And you can purchase a good book for gate, in that books you will find 10years  question papers and also includes your mock series.

Nowdays, many free apps has also launched so you can access your mock test paper from there also.

All the best.

I dont have much idea about the websites where you can get free mocks, but still you can visit MADE EASY, GATE ACADEMY, GATE FORUM, ACE ACADEMY & etc.