In Edgar Allan Poe's classic short story, "The Cask of Amontillado," Montressor seeks to avenge an unnamed earlier insult against him. Although both men are Italian, Montressor suggests that Fortunato, and "his countrymen," are apparently from a different area. The quote in question
"In painting and gemmary, Fortunato, like his countrymen, was a quack."
suggests that Fortunato, though a collector or a lover of fine art and jewels (gemmary), was basically ignorant in knowledge about them. Montressor's term, "quack," indicates a phony pretense of understanding. However, Montressor does give Fortunato credit for his discernment of fine wine, and this quality assists Montressor in his act of revenge.
Would you like the sentence broken down in terms of meaning or in grammatical terms (diagramming)? The two are connected, of course, and I'll try my hand at both, using a common method in descriptive grammar. This sort of thing is open to interpretation, of course, so maybe others will offer their versions.
The core sentence is "Fortunato was a quack." "Fortunato" (which is a proper noun) could be called the noun phrase functioning as the subject. "was a quack" could be called the verb phrase functioning as the predicate. This predicate contains the main verb ("be") in the past indicative as well as an indefinite article ("a") and noun ("quack") that together make up the predicate noun. In this core sentence, the "be" verb is working as a linking verb.
"In painting and gemmary" is a prepositional phrase and definitely works as an adverb in this sentence, but exactly what sort of adverb it is remains open to interpretation. It's not an adverb of location; maybe it's an adverb of manner or capacity? That's just a guess on my part.
"Like his countryman" is probably the most difficult part of the sentence to pin down. "Like" can be any of main part of speech, depending on how it's used. This phrase appears after and probably modifies the noun "Fortunato," so this phrase is probably working as an adjective. I'm not entirely sure but suspect that this is a prepositional phrase functioning as an adjective and modifying "Fortunato"; I am sure that "like" is a preposition and the words "his" (a possessive adjective) and "countrymen" (a noun) form the object of that preposition.
Of course, I may have completely misunderstood your question! If so, please let me know.
In terms of meaning, I might paraphrase the sentence as follows: Fortunato is representative of men in his country in that he doesn't truly know what he pretends to know about two types of art.