Kindly check out the link https://engineering.careers360.com/articles/gate-cutoff for detailed information regarding your query.

Gate cutoff marks changes every year. It considers various factors like syllabus etc before setting the qualifying the cutoff marks. For general category the qualifying marks was 45.46, for OBC it was around 47.07 and for SC it was 42.76, ST it was 31.62 ( Data is from 2016 ans 2017 )

You can add +/- (3-4) for 2018 cutoff