There's only a handful of climbs in the UK that are tougher than the south-west side of Bwlch-y-Groes. In my early racing days, I would make it up in 42-23 - that was seriously tough, but when your contemporaries are all similarly geared, it's not quite so bad.......... When smaller BCD chainsets able to take a 39t ring were introduced, I gladly fitted a 39; these days, since the advent of compacts, I don't consider tackling it without at least 34-28.
I'd say Bwlch-y-Groes is more leg-busting than all the classic Alpine cols and climbs. When one does the Telegraphe as the lower slopes, or prelude, to the full Galibier, that is a fairly daunting proposition (approx. 2,100m total elevation gain), but still easier than the brutal Bwlch and its 'mere' 376m elevation gain.