nemonium

The sentence misuses the apostrophe in the contraction of "them." With the "th" removed, the apostrophe should precede the word: 'em.

The apostrophe in "Nazi's" indicates the possessive case, as the plural of nazi — nazis — lacks the apostrophe.

A comma should separate the imperative clause from "GrammarNazis", at least for the sake of avoiding ambiguity. The sentence can imply "GrammarNazi's [sic]" as the subject, as in: [you Grammar Nazis,] go get them. However, "them" could colloquially modify "GrammarNazis," as in: [you, ambiguous person,] go get those Grammar Nazis.

The sentence also lacks terminal punctuation.