Digital Logic: Morris Mano (you will notice gate papers uses same terminologies as used by Morris Mano,
So I think Morris Mano is good option you will quickly understand the questions)
Algorithms: Not very much sure but I notice some questions are asked from back exercise given in "Introduction to Algorithms [Hardcover]"
Databases: very much confident that you much read: Elmasri navathe
Information Systems and Software Engineering: No idea..
Computer Networks: "kurose ross" is very good book, But some chapter are very good in "tanenbaum" and I again remember many questions asked in GAT papers are similar to questions in "tanenbaum"
Web technologies: No idea
Operation system: memory and computation related from "tanenbaum" and
algo e.g. process mutex from galvin