Talk at length (3)
'talk at length' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gab that I've seen before include "Talk at length, chatter" , "The gift of the . . . is ability to talk fluently" , "rabbit" , "Loquacity the gift of it" , "Natter" .)
(Other definitions for hold forth that I've seen before include "lecture" , "Speak lengthily, maybe preach" , "make a speech" , "speak in public?" , "Present" .)
(Other definitions for go on that I've seen before include "Continue" , "Hired thug" , "Foolish person - thug (US)" , "Harry Secombe or Spike Milligan, say" , "Harry Secombe, for instance" .)