This is a manga version of Gate: Jieitai Konichi nite, which also has an anime and light novel. I watched the 1st and 2nd season of the anime, loved the first and hated the second.

Reading this particular work left me with mixed feelings.

First here's a quality of art which is not on berserk level, but fairly high, and mature themes. However, with time it sorta degrades, and in places it becomes obvious that photos were used for backgrounds (the quality still remains quite high). On other hand, the series smell strongly of nationalism and leaves bad taste at some point. On occasion it reads as military hardware porn, as it displays more and more military stuff, people in uniforms positioned in different ways and showing off different angles. Then closer to the the 80th chapters it becomes nihilistically hopeless to the point where you start wondering whether there's even a point left in all those character dying, and where it is even going.

In a bit greater details:

Story: 5/10.

Fairly unlikely protagonist who is outshone by side characters. Side characters often have complex and believable motivations and at least to a degree resemble real people. Not that many anime steroetypes like tsundere blushing here and so on.

However, as I said the work strongly smells of nationalism, as the protagonist's army largely is displayed as some super-moral saviours completely superior to the magical civilization, and that, after a time, gets annoying and leaves bad taste in mouth. Protagonist in particular has definitely leaning towards being a jesus-kun, or at least a saint-kun, and is quite unlikely as he is.

There is a fairly strong lore in the world as well, with its gods, traditions, and so on, but the manga loves throwing darker tropes like slavery/sex slavery, prostition, abuse and so on in heavy handed and awkward manner.

The antagonists in the series are also fairly weak and act in non-sensical manner often. It isn't as bad as in some other works, though.

Art: 7/10.
Quite detailed, and even though it degrades in later chapters, it remains quite detailed. In later chapters around 80s I started noticing photo backgrounds and possible image reuse, but aside from that it remained above average throughout the series.

Characters: 7/10.

Weak main character with tendency of acting as an overly moral saint and otherwise being faceless/character-less, strong side characters with their interesting motivation, and insane villains that are not very believable.
Then there's cannon fodder characters which are not exactly logical at all times.

The villains have questionable motivation, and are somewhat hard to believe, although they never quite reach the level of a complete laughing jester lunatic. But they're close to that and are heading there.

Main character made me wonder if he was some sort of self-insert or wish fullfillment, but perhaps I am overthinking it.

Enjoyment: 5/10.

This manga gets quite annoyuing when it starts leaning towards nationalistic stuff, starts displaying might of JSDF military, superior morality of invaders, I mean liberators, or attempts to make protagonist act as a super-moral saint of sorts.

The work also gets annoying when it starts producing villains and horrible situations out of nowhere, although there are works that do it worse.

Oh, and that amazing scene with the assault on the almus hill is not in the manga. It was an unique thing of the anime.

On other hand... lore segments, side stories and the like are interesting.

But overall due to shortcoming it is a "meh" experience that did not make me sit at the edge of my seat.

Overall: 5/10.

It is worth checking out for good art, and events that weren't covered in the anime, but it has different feeling to it compared to the anime, and more heavily pronounced nationalism in it. That makes the experience less entertaining and hampers the work quality.

That's how I see it.