Gunk is a gloppy, sticky substance, like the gunk you scrape off the soles of your shoes or the gunk at the bottom of a pond.
Gunk is an American English slang term that was first used around 1950 to mean "viscous substance." The word comes from a liquid soap made in Massachusetts starting in the 1930s that was also called Gunk. If you buy Gunk today, it's a cleaning product used for car engines (and sold by a company known as GUNK). In general though, gunk is a great word for anything messy, slimy, or grimy.