Depending on context, Guy's could be singular possessive noun or a contraction.
Examples:
Singular possessive noun: That guy's hat is goofy!
Contraction: That guy's goofy! (guy's is a contraction of the noun guy and the verb is.)
part of speech
The part of speech for this particular word is a noun.
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath
The part of speech for "explicit" is an adjective.
The part of speech for "twirl" is a verb.
The part of speech for "unfamiliar" is an adjective.
The part of speech of "came" is a verb.
The part of speech for "lecturer" is a noun.
The part of speech for "indefinite" is an adjective.
The part of speech for "buried" is a verb.