Dear Yong Peng,

'Gerund' is actually is a misnomer for this Pali construct.

I would it is rather an adverbial participle or at least 'absolutive'.
In Ukrainian and Russian there's an exact equivalent of such
construct - 'deeprichastije'.

(See more on Russian grammar at
http://www.russian.unimelb.edu.au/students/parts.html )

Judging from the Russian correspondences, I suppose that in ancient
Indian -aya absolutives were present time, and -tva absolutives were past time.
Yet in Pali they both can indicate either present or past time.
Yes, such constructs form subordinate clauses. There's no direct
equivalent to them in English.

Metta,
Dimitry