What is the cutoff of XAT & how many marks are required to score 99 percentile

All depends on lod,in XAT 2020 28 was sufficient for 99%ile whereas in xat 2019 it was around 40.Cutoffs are usually 95 overall for bm and 92 for hrm along with sectional cutoffs for each section.

Do these marks contains gk as well?

40 is way too much. In XAT 2020, above 30 was enough. In case of easy exam, 36-37 should be safe.

minimum marks for 99 percentile is 40. It may be more depending on exam difficulty level

42+