I think Gabbert has more talent I'm guessing Arians trusts Stanton more in HIS offense because he knows it. I do hope at some point we see Gabbert get a few starts but I don't expect much.
in 2015 he played 8 games for the 49ers, I think he started 7 of those might have been all 8. He had his best year ever, first time over 60%, best TD to INT ratio(10-7) but it was still very apparent he just wasn't an NFL starter. His year playing for Harbaugh was pretty telling that was a very QB friendly offense it was run based and he was not asked to win games just don't lose them. 8 games he put up decent numbers but he didn't make big plays and he appeared very risk averse. Maybe that was the coaching who knows.
But he has the size, he can move, and he's got a strong enough arm so it would be good to see him get a few starts here to see if he can play in Arians' system that is the opposite of risk averse.