No bullshit, long-winded explanation of what the manga is. If you want that, the synopsis does a pretty good job and the plot summary on the wiki is good too. I'm also not dividing this into sections, just because it feels pointless when most manga mesh both character and plot concepts together to where tearing them apart makes it harder to analyze.

To start off, the title might give the impression that this will have a bad/average score, but if you quickly scroll down you'll notice a rather high score. Because, regardless of how bad I think the direction went halfway in, the plot writing actually improves, and the characters, especially Alita herself, are much MUCH better than before. I think the score would be lower if there wasn't a sequel, but for now, we will put that aside until I write my review on Last Order, which IMO is so far the best entry in the trilogy.
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Gunnm starts off on a rather good note, showing off the art and the Scrapyard as a whole, a scuffed city that provides for the people up above on Zalem. This whole beginning bit has some questionable writing and pacing, especially the villain Makeku and the romance with Hugo, but for the most part, the characters are fine. This is probably where Alita is at her worst and most "submissive" since she is constantly being led around by Ido and his possessiveness, and also where Ido is at his worst for, let's say, being left to just being a father figure which wants Alita to stay out of fighting. Then Gunnm pulls a weird move and makes the romance with Yugo turn from a subplot into a real plot, which doesn't end up developed and not really believable, and honestly this "arc" is a mess and well-deservedly short so we are just going to skip over it.

Alita Motorball Arc

The next arc has Alita in the Motorball games, and for fights this might be the strongest arc, and where things start to go uphill for the manga as a whole, with the introduction of Jashugan as an "antagonist" which is more of a rival character. Here Alita becomes completely independent and not reliant on Ido, and where she finally accepts her true nature as a fighter after facing Jashugan. There's not too much to say here, but this is where Gunnm hits the peak world-building-wise and its a steady decline from here. The characters and the plot still get better than here, especially with Alita herself.
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After the end of the Motorball Arc (takes place in the Scrapyard), Gunnm sets up the main antagonist of the series named Desty Nova, who apparently is behind everything that has happened to Alita thus far. Now as much as I do enjoy Nova's character in the later entries as he takes a sort-of more backseat role, this is the absolute worst way to introduce an antagonist, because up until this point everything was both BLACK AND WHITE, while now it's BLACK VS WHITE.
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In more simple terms, Desty Nova is set up as an irredeemable mad scientist who is all the worlds evil and Alita is the undeniable good. The previous antagonists for the entirety of the Scrapyard always had their own motivations, ambitions, reasons, or complete explanations as to why they believe in those things, and why that clashes with how Alita views the world (Makeku because of the way he was raised and his conditions, and Jashugan because they were similar but had different goals). Nova is just the mad scientist trope. THAT'S IT. It's stupid, and the shitty part is Nova is the antagonist until the end of this entry. THE ENTIRE TIME.

The logical next step would be to search for this Nova, and this takes us to the Badlands and the beginning of the TUNED Arc. Sadly, aside from the HUGE amount of foreshadowing given to us by the ruins in the Badlands and the general location of the Badlands, there is no more world-building aside from that since the Badlands consist mostly of desert with a few scatered towns everywhere.
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The entire lot of characters introduced here are, as I said before, an absolute improvement, except for the romance between Alita and Figure Four. In general, the romance with Alita throughout the whole story is not very well done and honestly would have been much better without it entirely. The plot here is very good, as I think Den is ambitious in his goal and the Barjack organization being behind him is the icing on the cake (I'm a sucker for labor revolts, so this would obviously entertain me), and not only that it completely clashes with whom Alita is sided in this arc creating conflict. There are a couple of plot-twists in this arc, one left a bad taste in my mouth and the other wasn't really that impactful but cool to see and revealed more information about the unknown world of Zalem.

Though, sadly, as stated in the title, it gets tunnel vision. After this antagonist is taken down, Nova is somehow involved again and stays as the antagonist until the end, not leaving any room for more world-building beyond the Badlands, for Zalem, or even what was left of the Scrapyard/its origins. The original ending makes this worse by rushing things with exposition dumps, and not leaving any time to process the information. Last Order gladly retcons this, but the canon ending is just abrupt and isn't conclusive enough.

That's it. It's an 8, simply because most of the major problems I had while reading it I could easily ignore because the plot-twists, characters, and art were stellar, and also that it's sequel fixed all the problems I had with this one.