As a Garen main, I'd say I'm probably biased in my opinion of this but in my experience, I generally win lane against Darius.
I'd say its a skill matchup, but still slightly in the favor of Garen. Here's a few reasons why...
Darius is gated by mana, Garen only by cooldowns.
Garen has built in sustain, so he can trade, sustain his health back and continue to trade. Darius doesn't.
Garen benefits more than Darius from armor and MR. He gets an additional 20% for everything he builds, Darius doesn't. Its worth noting that Darius does have built in armor penetration, but Garen can afford to build armor and still do decent damage because of his high base damage, whereas if Darius builds armor, his damage will be lacking in comparison.
Garen can trade with Darius and get him low enough for a flash Q+E+R combo and deal the full damage of his ultimate. Darius needs to apply multiple stacks in order to deal the most damage possible. Point being, Garen's ultimate can be used immediately, where Darius needs stacks to be relatively effective.
All of this aside, Garen can still easily lose lane to Darius if he doesn't trade properly and vice versa. Its a skill match up at the least.
In response to the actual topic, your jungler is a fool and should gank regardless of the matchup. What you should really be telling him is "If I can win so hard in this lane, why not gank and make it
-teed so that I can continue to snowball even harder?"