I suppose if someone went out of their way to look for and examine this type of drinking glass, then examples with gadget marks might be found more often - over the few years I did look I only found a single glass showing it had been held in the gadget.
Am assuming that all examples with scars, ground/polished pontil depressions and Y and T marks would have been finished on the pontil rod, not the gadget. Have to say that on my one 'gadget' glass, the finish on the foot is rather crude, and no idea if this invention was something that was used in the U.K. only, or whether it found its way onto the Continent.
If you read Wilkinson, he looks to be saying that by c. 1830 the gadget appears and the pontil mark disappears - he also offers the same date to indicate the change in method of applying handles i.e. top down giving way to the reverse.
IMHO I think we have to read his comments using some latitude of interpretation - I know others have used a later date for the handle change, and whilst we don't see massive scars on the undersides of high domed feet, I'm sure there are lots of glasses made post c. 1830 through the C19 where scars of some size can still be found. Obviously Wilkinson had a massive knowledge re glass but was perhaps a little too brief with his explanations.
He also indirectly appears to offer an explanation as to perhaps why we don't see the gadget mark on feet as frequently as we might imagine - he says .. apparently, c. 1900, the workers would use asbestos string - inside the gadget jaws - to create a softer grip on the foot of the glass - and Wilkinson says this 'eradicated' gadget marks on the glass.