Incessant talker (6)
(Other definitions for gasbag that I've seen before include "One talks at length on boring topics" , "Person who talks too much - or a balloon" , "Loudmouth - or balloon?" , "goes on and on" , "Garrulous type" .)
'incessant talker' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for motormouth that I've seen before include "Non-stop chatterbox" , "Someone who won't shut up" , "Constant talker" .)