Quick fix for leaky garage roof?

Any decent DIY store will have tins of roof sealant, which is simply painted on. Some types have to be put on in the dry, others (more expensive) can be applied in the rain if it's a real emergency. I bought a tin of the 'apply in the wet or dry' for about £12.00 a few weeks ago with enough in it for a square metre of coverage. It did what it said on the tin, and we've had no more problems. Of course you would have to remove all the gravel first before the paint can be applied which if not done carefully can actually damage the waterproofing further. Another point is knowing exactly where the leak is! I notice you said 'leaks'. If the roof covering is quite old and has a lot of areas that have deteriorated, then you may find that the best solution will a replacement covering, rather than having to constantly patch all the time.
Good luck with the leak finding!