1.You can apply for GATE 2018 using the marks sheet for the final semesters (any one) of your B.Tech. However you will have to clear your backlog by August 2018 to be able to avail admissions in M.Tech.2.Yes, that would be enough. As per the eligibility criteria of GATE 2018, you need any one of the following documents while applying for GATE 2018 - degree certificate/ provisional certificate/ course completion certificate/ professional ceritificate/ membership certificate issued by the society or institute, you have completed your last education.4.You can upload a copy of your final year marksheet OR a declaration from your college Principal (or other competent authority) along with the signature and seal.3.The candidates with backlogs (failed subjects) in any of the subjects in their qualifying examination will have to upload a copy of any of the mark sheet of the final year issued from the institute where he/she was studying. You can check the further information with the given below link. Https://www.engineering.careers360.com/articles/gate-eligibility-criteria