Gordo Slimes are a rare phenomenon that occurs when multiple slimes of the same type congregate. The result is that the slimes seem to merge together into a much larger slime that is seemingly incapable of movement.
A gordo will eat until it bursts, causing all the slimes to separate once more, as well as revealing whatever object it was that caused the gordo to form in the first place.
Gordos pose no real risk to ranchers, they can only sometimes be a nuisance in that they are hiding something a rancher might need. So if you see a gordo, feed it!
Gordos produce no plorts on their own, however the slimes they produce after bursting do like any normal slime.