Hi aspirant, its good that you are going to prepare for GATE from 2nd year.
First of all you should check the syllabus for exam as some topics are removed from syllabus in coming years.
Now the books you should refer are:

Operating System: Galwin is best book. Others are Tanenbaum, William Stallings and lectures of NPTEL.

Theory of Computation: Introduction to Automata Theory by Ullman

Algorithms: Cormen

Computer Networks: Tanenbaum

Computer Organisation: Carl Hamacher

Database Management System: Korth

Compiler Design: Ullman

Digital Logic: Morris Mano

Practise more number of previous year questions. Attempt test series.
If you attempt test series, you'll surely score higher.
If you have any more queries, comment down below.