“Hermes” (vocative) actually stands outside the γαρ clause, which is parenthetical, explaining why it’s Hermes he’s addressing (i.e. in his capacity as message-boy).
“Hermes—for you’re the messenger in everything else (in turn) too—(tell Calypso to send him home.)”
It’s a common use of γαρ, strictly speaking elliptical, explaining why he’s said what he’s just said, especially common with vocatives:
“Hermes—(it’s you I’m talking to) for you’re the one who carries my messages—, tell …”
The next line, with its imperatival ειπειν, continues directly from Ερμεια.