Any kind of drippy, yucky, gunky stuff can be called goo. Some people love tapioca pudding, while others feel that it's nothing but lumpy goo.
The filling in a donut is one kind of goo — custard or jelly that oozes out when you take a bite. An unidentified blob of slime on the sidewalk or the glistening trail left behind by a slug, are other, much less appealing kinds of goo. Goo was coined in the US, but its origins beyond that are unclear. Many experts think gooey came first, possibly shortened from the now-obsolete burgoo, or "porridge."