It will take a few days for the gunk to be preened away, by the chick. If you absolutely cannot handle the wait, you CAN wash it with very warm water, holding it very carefully in your hand under a low flow from the sink or bathroom faucet. Keep its head out of the water, and with one finger, sort of "shampoo" the feathers without any soap, just use the warm water.
Then towel dry the chick and put it either back into the incubator or directly under the brooder lamp to dry out. (I recommend the incubator, if yours is a forced air type with a fan.)