You can check the important topics below for mechanical engineering for GATE

Manufacturing - Orthogonal cutting, Tool life equation problems.

Thermal Sciences - Laws of TD, Properties.

Fluid Mechanics- Velocity potential, Flow through Pipes, Boundary layer theory.

Heat Transfer - Numbers in convection like Pr No, Gr No, Re No, etc.

Theory of Machines & Vibration - Pairs and its applications, Linkages, Inversions of single and double slider crank, Free damped vibrations

Industrial Engineering - Queuing theory, EOQ problems, Inventory control.

Engineering Mechanics - Structural analysis, Work and Energy, Law of Restitution

Engineering Materials - Effect of Alloying elements, Defects in the crystal structure.

Strength of materials - Joints, Design of gears, and bearings

Engineering Mathematics - Every topic is important.

Good Luck!!