The answer to the question "what is the function of the verbal in the sentence: My goal is to skydive over the Grand Canyon" is It is used as a noun. The verbal word is "to skydive" where the verb skydive is used as a noun, or thing, the activity of skydiving. It is not used as a verb as the verb would be ongoing activity, an adverb describes a verb and the adjective would be describing the noun.