Besides the FIIs pulling out, there is dollar demand also from the State-owned oil importers. So long as the global financial turmoil persists, the central bank will keep offloading greenbacks.
Its foreign exchange reserves kitty is already slimmer as a result. Whether this gathering trend marks a sudden reversal of the phenomenon of rising foreign exchange reserves bears watching.
But one thing is for sure. There's not much that can prevent weakness in the currency unit in the period ahead.

Could you explain the emboldened parts to me?
First of all, I confused what "this" refers to here? Also, what they mean by "gathering trend"? If possible, please rewrite the whole sentence "whether .. bears watching" with a simple Engish.

 The first 3 sentences in the paragraph explain why the amount of dollars (the reserves) in the central bank is decreasing. "Pulling out", "dollar demand" and "offloading" all mean that the value of the dollar account is going down. The FII is taking dollars, the oil importers are taking dollars, and the central bank is selling dollars. .

"This gathering trend" refers to the decline in the reserves.  The trend is that the decline may continue or even accelerate.

This decline is contrasted with the increases in the reserves that were happening earlier (the "reversal").

"Bears watching" means that you should pay attention to this trend to see if it continues.