That sent me scurrying off to my records. Fastest I've ever done 100k was a bit of a cheat. I knew there was a howling wind from the West. So I took a train to Gatwick and rode to Whitstable with the wind at my back. 4:39 moving and 5:24 overal for 114 km.
Slowest was ... er ... last weekend. My rain and hills fest on Saturday 6:26 moving with just under 90 mins of stopped time for bang on 100k. With some of the routes I have planned I expect I will break that record in the near future!
Incidentally, I looked up the term Gran Fondo and it seems to be used in various different ways. So it's not surprising I was never sure what it meant.
The official Italian meaning - and it's an Italian phrase so they get first dibs - is a mass-start timed event of 120km or more. But it's also used more loosely in various places to mean roughly the same as "sportive" with no strict definition. And there's the Strava definition of 100k that you mention. And there's the UCI Gran Fondo World Series, which seems to be a mega serious racing thing
UCI-sanctioned races ... focusing on both very competitive riders who either just miss the skills and talent to become a Pro Elite rider or who still want to compete at a high level at a later age (
link)
. But I'm not sure of the distances. So is it a timed event or is it a race? Is it 120km or more or less? Who knows?
https://epicroadrides.com/cycling-blog/what-is-a-gran-fondo-sportive-cycling-guide/