In My 6th Semester 1 have 1 backlog Then I eligible For Gate 2018??or Not

Yes, you can apply for Gate but you will have to submit the documents before Gate admission. Also, candidate with backlogs (failed subjects) in any of the papers in their qualifying degree will have to upload a copy of any of the mark sheet of the final year issued from the Institute where he or she is studying. It is to be noted that internet downloaded marksheet will not be accepted under any circumstances.

Hope this helps!!!