I'm sure the ring-necked snake will eat geckos though I've never tried it myself. Brownsnakes eat primarily invertebrates and in the wild the specialize on worms, snails, and slugs. I'm sure you could get it to take other foods by scenting them at first but why bother? I think it's easier to get ahold of those other foods anyway or at the very least, cheaper. Browns stay fairly smalll anyway. It might bulk up a little bit but likely won't ever be as bulky as oh say a boa or something.