"Gullet Butter" refers to a thick soft yellowish or whitish viscous substance, especially when used as a lubricant, that oozes any path or channel or duct through or along which something may pass.

This term is generally used when something needs lubricated.

"cum/precum from the opening at the tip of a man's penis"
"snot from the nasal passages"
"Saliva from the mouth"

A casual invitation / threat used by an individual that is not afraid of fucking someone up.