If you ever notice that your rainwater gully is overflowing when, for example during periods of heavy rain fall. You can assume a high probability that the gully is blocked with leaves or other debris.
To unblock, take off the grill and using a gloves start to hand remove all the debris. Clear the bottom of the gully pipe with a length of wire, and pour some disinfectant down the gully this will help to remove any foul odors.
If the flooded gully appears to be clear of debris but still continues to flood, then try locating the blockage at the nearest inspection point.
Try using a high pressure water hose insert same down the gully this can be an effective way of forcing loose debris.
If none of the above works, call your local plumber.