Hello,
Ofcourse you can. You can apply for Gate to pursue masters irrespective of whatever branch you have a degree in. You can obviously apply for GATE. architecture with an MSc in Computer Science.

See your BSc wont be considered for GATE or even M.Tech. You need to complete MSC Next, percentage doesn't matter to appear for GATE. You just need to have passed MSC But for M.Tech admissions or PSU you will require at least 60% in your MSC So study such that you have above 60% in your MSc since you will anyway be taking admissions in it.