Fork oil seals should be lubricated regularly with light oil to stop them drying ot and cracking. BUT as the GS has a separate damper/spring assembly on the front, I'm not sure there is anything in the fork leg worth worrying about.
I regularly clear the trail crud out of the top of my mountain bike forks by adding a few drops of 3-in-1 oil and pumping them and wiping the stanchions until theres nothing but clean dry oil left.
I think a leaking fork oil seal will make a bike fail its MOT test but again I'm not sure the telelever front is subject to the same check (at least, perhaps not at a BuMW dealers).
I'd just clean it off and keep an eye on it.