It is just wear from when the gun recoils when shoot or you rack the slide. There isn't really any way to keep it from happening. But I put a light coat of oil on the outside of the barrel of all my Glocks.
When surfaces rub one another, what gets worn away is only the material which impedes movement. This is what leads us to value objects which are "broken in".
Whether an engine, firearm, or any assembly of moving parts, they all work better once the "stiction" is removed.
TL, DR... Those wear marks are making your firearm more reliable. Don't sweat it.
That's normal. The barrel rubs the slide a little when it tilts during cycling. Make sure to put a skim of oil on that part of the barrel, but it's normal for the finish to wear off in a few hundred rounds like that.
Your finish is already worn off there is not a whole lot you can do about it. I usually leave a coating of oil over the barrel and slide cut out for the muzzle to keep it extra slick there to help prevent wear. It just means you have actually shot your pistol.
Too late to do anything about it now, but more oil will prevent that, or at least drastically slow its formation. I had a G26 with thousands of rounds through it that had no such wear simply because I kept it oiled. Glocks have a pretty durable finish. The barrel locks up against the opening in the front of the slide when in battery, and then unlocks and slides against it when firing, so there's a lot of metal-on-metal contact there. That sort of contact demands lubricant.
Those marks are normal for Gen 5's. Gen 4 had a different coating that didn't wear off as quickly but that happens relatively quickly on a Gen 5. Nothing to worry about; it is only cosmetic. Just lube it as normal and it will be fine.
The real reason is because it's a Glock, nothing but cheap plastic and fanboy hype!
OK, got my anti Glock dig in for the record.
As everyone has stated so far, completely normal and nothing to worry about. A little oil on the (just a wipe) should always be applied to the barrel. My M&P's (also plastic) show the same wear.
Glocks are good firearms, just not for me.
Clean it, lube it, respect it, and carry with confidence!