Phage gives AD. He also probably has AD from his runes and masteries. Probably not much, but Garen doesn't really need much for early damage.
Ninja Tabi helps you against auto attacks, and honestly has a pathetic amount of armor. You had no other armor or health from items. Just so we're clear about this Garen's spin does not count as auto attacks.
That is why he chunked you down so fast, because you're relying on meager defenses against a champion who, early on, is pretty much just movement speed and damage.
You didn't die, but Garen still decided to teach you just how effective your stun and boots are against an AD caster. I suggest you take that lesson to heart.