I have completed with my Gate syllabus, should I start the mock papers or should I revise the syllabus once again?

If you have already completed your syllabus then I would suggest you to give mock papers as much as you can. It will boost your confidence and will also let you know about your preparation level and will show all your weak areas. And lastly, I would say after completing the mock paper always consider the analysis report of the test properly. Good Luck!!!