Depending on the roof design the pebbles can also be to moderate the rate/way water drains from the roof. But evidently something is going wrong if they are blocking the drains. Probably it is all the dirt and weeds in between, or a filter to stop them going in the drain is missing. It may just be that they haven’t been cleaned often enough.
I’d check with a builder or roofing contractor before I disposed of them as it may cause other problems if they are there for a good reason.
If you are in NSW and can’t find your purchase contract, you can call Dept of Fair Trading Strata section and they can bring up your Strata Plan and tell you what your lot consists of, and what is common property. I seem to remember that when I consulted them they rang back after they ordered up the file, so it may have involved a day or two delay to get my answer.
You can probably find information about the levies you pay compared to others from financial information in the Owners Corporation AGM agenda and use that as Tess has described.
As others have said it seems unlikely it is your responsibility, but check to be sure.
Roofing contractors can do the work for you even if you don’t have a fence or balustrade – they will use a harness if necessary.