1. Grab a lysol wipe and clean the gunk off and dry it immediately with a paper towel
2. Buy a little bottle of Bore-Oil at your local music store that does violin lessons and the like.
3. Apply a drop or two of bore oil on the fretboard and spread it with a q-tip.
4. Let it dry overnight and wipe the excess off which will be very little.
Done.
Going forward, get in the habit of wiping the fretboard with a clean microfiber towel after you're done using the guitar and you won't have to deal with gunk.