In questions, γάρ asks for confirmation of a preceding statement, or expresses assent or dissent; asks whether an act before mentioned was not reasonable; asks a question prompted by some form of emotion; and serves to indicate transition, etc.
a. In questions γάρ often marks surprise or indignation, and may frequently be translated by what, why, then, really, surely. Thus, ταυτὶ_ λέγεις σὺ στρατηγὸν πτωχὸς ὤν; ἐγὼ γάρ εἰμι πτωχός;
b. Brief interrogative formulae asking for confirmation of a preceding statement are:
ἦ γάρ; is it not so? surely this is so? (cp.
οὐ γάρ; is it not so? often in indignant questions; when not standing alone, why not?
πῶς γάρ; πόθεν γάρ; imply that something is impossible (often of surprise). Cp. πῶς γὰρ οὔ; in negative rhetorical questions.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].