Ohhhh! Digital electronics (DE) is one the most simple subjects in EE. Let me show you how to make DE from scratch!
You need to first have DE reference books! M.Morris Mano (Digital Design 5th Ed(solutions on the web)) is a great reference book for answering difficult questions! A.Anand Kumar (Fundamentals of Digital Electronics 2nd ed.) is the best book to read the text!
For logic families, read R.P.Jain (Modern Digital Electronics).
Start with the basics by checking out NPTEL lectures S.Srinivasan (IIT-M), gives a lecture series on DE! He is undoubtedly the greatest teacher in DE!
Watch lecture series by AmitavaDasgupta (IIT-M), for logic families!
This would be enough to study DE for GATE!
All the best!
Happy Learning! 🙂