This is quite possibly in my top 3 books of all time. I could talk about it for hours, so brace yourselves...this review is going to be a long one.

The Story Itself
This is my second time reading Gemina, and I'm just going to start off by saying that everything hits differently on a reread. While there is a certain level of amazement that comes with the first read-through that can never be replicated, there are so many tiny details that hurt so much more the second time around.
As the second book in the Illuminae Files series, the story takes place at the Heimdall jump station (basically a space station with a wormhole in the center) that is attacked by the evil corporations responsible for the terrorism from the first book. We meet Hannah Donnelly, the captain's "perfect" daughter, and Nik Malikov, a boy with a criminal background, as they fight back against BeiTech Industries before they can kill thousands more. The plot twists are wonderfully done (I somehow forgot a bunch of them?), and the ending is just...you have to read it for yourself, but it is quite possibly the coolest thing I've ever read in a book, and I'm still not over it. The subplots and action scenes all tie together to form an entertaining and heartbreaking story of rebellion.
And the characters? I love each and every one of them (okay, not the BeiTech people). I love Hannah defying the stereotypes around her while simultaneously proving to be an incredibly strong character. I love Ella's snark and how she doesn't take pity from anyone. I love the relationship that the main cast has with Isaac Grant, which sets up for the last book as well. I like how AIDAN plays into the story, though, as I said before, I'm not as big of a fan of him as I was during my first read. Nik is my favorite, of course, because he's so complex, and also, the tragic backstory/sarcasm combination always wins my favor.
I will always recommend this story to all sci-fi and action/adventure fans or anyone looking for stories that mess with formatting because this truly isn't a book; it's an experience.

The Audiobook
So I'm currently rereading this whole trilogy with the audiobooks and physical books side-by-side, and I'm loving it. The sound effects and the voice actors do a phenomenal job of bringing the story to life.
As I said during my Illuminae review, I would not recommend reading the audiobook alone because there are so many visual elements that are skipped out, and I think that's especially true with Gemina, for reasons I'll explain in the spoiler section of my review.
Firstly, the sound effects. Amazing. The one firefight scene gave me literal chills with the sound effects combined with the combat instructions from the antagonists overlapping with pleas for mercy. It felt like I was really there, and the sound effects throughout the rest of the book gave me a similar feeling.
Now, for the voice actors. This section is purely subjective, and I just want to put it out there that I'm not a typical audiobook reader (I've only read the audiobooks for this series), so I apologize ahead of time. The actresses for Hannah and Ella really nailed their characters. They sounded almost exactly how I pictured the characters, and Hannah's actress did a great job at the super emotional bits. I also think Falk's actor did great at really bringing out the negative parts of his character. The two actors I struggled to adjust to were Jackson's and Nik's. Jackson is noted to have an accent in the book itself, so I shouldn't have been surprised that he was played by a British guy, but it sounded like he was straight out of the 1800s or something...I liked it by the ending, but it took a while to adjust. The same goes for Nik: his voice didn't match how I pictured him at first, and even though I liked the sound of it on its own, it took me a while to match it to Nik as a character (though I was definitely a fan by the end...especially with his dialogue with Hanna).

Spoilers
I usually don't do spoilers in my reviews, and I'm not officially using a spoiler marking, so read at your own risk. I just have a few things I really want to say that have spoilers in them.
1. Jackson's actor lost his accent halfway through? Spoiler alert: Jackson is actually the secret operative and he's evil and whatever...but after the reveal, he suddenly lost his accent and I don't know why, but it bothered me at first.
2. I have mixed feelings about the audiobook formatting with the parallel universe stuff. For the section where Nik and Hanna are both answering Kady's questions at the same time, the audiobook had Kady say her line and then Hanna answered, then Nik and I thought that really captured the parallel universe feel. However, at the big ending section with the side-by-side parallel universe stuff on each page, it read the left page and then the right. I think it would've been cooler to read the top lines of each page, then the next line, and so on (or split it into smaller sections) because it kind of ruined my immersion in the double universes.
3. This has very minor spoilers for Obsidio, so don't read on if you haven't finished the trilogy yet. The analyst with the 7213-0089 ID is canonically confirmed to be Nik reading. Two points about this: a) they still have an Australian guy reading it even though Nik's actor isn't Australian and b) ... Nik is narrating his own death (sort of). Looking back on it now and the sections the analyst narrates...Nik is literally watching his other self die in Hanna's arms...it hurts, okay? It hurts a lot.

Conclusion
Once again, I apologize for the incredibly long review, but I still think there are a dozen more things I could say about this book.
This series really is one of my favorites of all time, with this book being the highlight of it all, in my personal opinion. I will always recommend it to anyone who dares to ask for recommendations from me.
5/5 and a well-earned spot on my god-tier shelf