I would suggest you to go through the below link as it will be providing you the complete information about  "How and what to study?"

https://engineering.careers360.com/articles/gate-how-prepare-for-chemical-engineering-ch

Good Luck!!!

Some of the books for GATE Chemical engineering are:

• Mass Transfer - Robert E.Treybal
• Heat Transfer - Unit Operations of Chemical Engineering by McCabe , Smith and Harriott
• Fluid Mechanics - McCabe Smith
• Process Calculations
• Thermodynamics - JM Smith
• Plant Design and Economics for Chemical Engineers, Author's Name: Max Peters, Ronald West
• Arihant's Chapter-wise Previous Solved Paper

These are some must books to have for GATE chemical preparation. These books shall be easily available on the book stores and even online.

All the best.