Gold ETF is cheaper as compare to GOLD MCX. Then why to invest in BULLDEX?

Looks like you are not aware of the basics of BULLDEX. To learn about it, watch this video167. We also did a webinar about this today. BULLDEX is a cash-settled futures contract that simulates the performance of precious metals Gold & Silver in the 2:1 ratio (Approx). ETFs are a completely different thing.