Hi Krishna, 

During GATE preparation, the toughest challenge is to find the right books to refer from. 

There are a few books here for you : 

Operating System - Galvin

Alogrthym - Cormen

Architecturen - Hamacher

TOC- Theory of Computation - Ullman

Also you can check out last year cut off, and you can refer to last year papers for preparation

I hope this helps you out 

All the best