Sportive (Gran Fondo) Finder

Unfortunately this sort of reverse search is unlikely to work. It relies on there being some sort of central database of events and their climbs, possibly an international one. While I can find a few fairly complete lists of UK events (in normal years) these lack sufficient information unless the climb is in the name, and you have to look at the individual event information to find out where it goes. Even then, only the most significant climbs tend to be listed, though those are probably what you're looking for.

But there aren't that many events, so the reverse search may not be needed assuming you can find lists at a national level. If you've got a time period of a few weeks, you can probably check all events in mountainous regions in a couple of countries without too much effort - after all you'd only need to do this once per year in spring, and many events run the same route each year. In fact googling the names of the climbs you particularly want to do will get you started.

It would be nice to have such a list, but someone would have to spend quite a bit of time on keeping it up to date.