Jada, the only reference to "gravlings" I could find was from the Showtime series "Dead Like Me". I never watched it, but surmise it has a paranormal theme. From the gist of what I found, a gravling is somewhat like a Grim Reaper figure, only on a small scale, and are capable of causing humans to have fatal accidents. This information was only in regard to the television series, so I don't know if it is a case of the writers lifting a folklore term and applying it to suit the needs of the series. I also checked with my husband to see if he had ever heard of the term and he thought it sounded familiar but couldn't quite place it. He wondered if it was a term from the Dungeons and Dragons game.
Can you give us a little more detail as to what you saw? When you mentioned small and grey, it almost reminded me of alien encounters I've read about. It is interesting that you and your mother saw the same thing.