If you meant 6th semester then you might be able to sit for GATE but there is one problem.
You will have to submit an acknowledgment that you are currently a final year student and have no backlogs. That was the scenario in 2015,2016. This is signed by your HOD.
You should further investigate this. A form is available online for previous year if you want to check.
Better be sure yourself, than believing in strangers words.
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
Good Luck!
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