Urgency can't be the only definining feature of a stop gap aircraft.
Yep, it isn't, the Lanc MK.II is also a good example of a stop-gap that was built to a foreseen development, but ultimately wasn't required. Although in terms of the countering of an opponent's superior development in an evolutionary function, then urgency is most definitely required, particularly if the conditions for whatever the stop-gap was being built actually took place, like in the case of the Boomerang and Spit IX.
The IX and XIV are types of stop gaps until the definitive developments were ready (ie VIII - definitive 2 stage Merlin Spitfire, and 21 - definitive Griffon Spitfire). The IX and XIV were shortcut developments.
Oh, alright, I'll give you the XIV, although the term 'interim' (definition meaning between one interval and another) is probably more appropriate to describe the 'XIV, rather than 'stop-gap'. Indeed, the 'XIV was, as described as "A useful interim type" to the Mk. XVIII.
I'm also aware of the development of the laminar flow wing - even the VIII was subject to investigation for a laminar flow wing and I vaguely remember reading something about it being intended for the XVIII, but was first fitted on the Mk.IV DP851 for the '21, which is why I mentioned it. My bad about the XIV engine though. Aaaand, the 'VIII was based on the HF.VII (the first recipient of the Sixty Series Merlin), which in turn was built from a 'V. The VIII had the 'universal' wing.
And herin lies the Spit IX's standout claim to being the most successful stop-gap fighter
Well said, Cobber; both the 'IX and the 'XIV were produced in larger numbers than their intended successors, too.