Yeah, I realize it goes way back, to the Phoenicians, or some other ancient script, but somewhere along the line it seems to me someone messed up.
Why would it?
Because they sort the alphabet in alphabetic order.
This makes my head hurt. 
Same reason why Q and U aren’t next to each other.
Have you tried Wiki for an explanation – which is actually quite simple and straightforward:
The letter 〈C〉 was the western form of the Greek gamma, but it was used for the sounds /ɡ/ and /k/ alike, possibly under the influence of Etruscan, which might have lacked any voiced plosives. Later, probably during the 3rd century BC, the letter 〈Z〉 — unneeded to write Latin properly — was replaced with the new letter 〈G〉, a 〈C〉 modified with a small vertical stroke, which took its place in the alphabet.
Je ne comprends pas. Expliquez moi, s’il vous plait.
So totally confused by the OP, yet intrigued.
Here’s the history of the Latin alphabet. Tell us what you’re confused about. And here’s the history of the Semitic alphabet while we’re at it:
A B C D E F H G I got a gal in Kalamazoo!
Add me to the list of puzzled Dopers here. The order is essentially arbitrary, so what makes you think that anybody ‘messed up’?
Maybe he thinks it’s based on the periodic table.
[H.G. Wells]
Makes perfect sense to me.
[/H.G. Wells]
Author, War of the Werds
Let’s prioritize here. If someone were to give me the power to rearrange the alphabet, the first thing I would do is put U and I together…
Are you confusing H (English aitch) with H (Greek eta)?
Are you saying it’s supposed to be Jesus G. Christ?
Why doesn’t M come first?
Yeah!
And even then, gamma is listed first.
Since I think the factual aspects of this question have been answered, let’s move this over to IMHO.
Colibri
General Questions Moderator
Let me tell you a little story.
Once upon a time, a long time ago, several of the letters were resting after a hard day’s work. Now there weren’t as many letters then as there are now, because this was a very long time ago. H, who was a tough, strong worker, was looking at J, who was as lovely back then as she is now. H was entranced by J, so he invited her to go for a walk along the stream.
Well, as time went on, H and J fell in love with each other, and eventually got married. A was the best man, and Z was the maid of honor. After H and J had been married for a couple years, they had a boy. And they named him I.
And now you know. . .
the REST of the story.