It will probably suck up some gravel, depending on how big the gravel is. The intake will get clogged if there are a lot of twigs or thick leaf stems in the mix. Oak leaves will also sometimes start a clog because they are thick and don't grind up as easily.They are of limited usefulness on a leaf cleanup. Useful, just limited. I am often glad I have it on the trailer, but it doesn't do much of the heavy lifting. Blowers, rakes and tarps do the bulk of the work and the Billy Goat sometimes helps clean up the mess.
Depends somewhat on how you are going to get rid of the debris. They will reduce the volume of the debris somewhat, taking up less space on your tarp, trailer or leaf bags. If you are blowing to the woods, debris reduction doesn't matter. Often I will blow leaves out of beds to the driveway or sidewalks. After removing much of that debris, the Billy Goat is good for sweeping the hard surfaces clean. The hose intake is useful for spot cleaning when you don't want to blow a small number of leaves around. Sucks leaves out of window wells quickly. I think customers like it because they see a guy vacuming their lawn - seems like you are being fastidious about your work. Conversely, they are not always impressed when they see you pick them up with a zero turn and vac bagger because they think they are paying a lot for a guy to just mow the lawn. They ignore all the blowing and raking that proceeds the Z work.