Yes you can write GATE as you main branch would be CS along with additonal subject as business system. The main paper you'll have to select is CS. It would be the same procedure for you as it can be for a CS student. So just apply for it being a CS branch student. You will have to cope up with the subjects your branch (CS business system) has eliminated from the CS branch's syllabus.

for more details. kindly visit the official website of GATE