This game is unironically one of the best games I've played in a while, as someone who's a huge fan of Touhou & lurks meme channels a lot, this game felt right at home. Fun gameplay, hilarious dialogue, a loveable original character.
The following will be spoiler free, because I think it's best to experience the entirety of the story for yourself.

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Gensokyo Odyssey is a Touhou Fangame, including a lot of the characters from the Touhou games (including Touhou 19, which is the most recent as of this review) alongside an original character made specifically for the game.
This game is a Visual Novel with a lot of humor in it's dialogues, referring to a lot of modern & old memes, it has slang, Touhou community in-jokes, etc. You will also have battles against some of the characters, and your way to fight them is using air hockey, you and the enemy both have your paddles, and there's a puck, you score into the enemy's goal to bring down their HP, and vice versa, the enemy will usually unleash attacks reminiscent of typical mainline Touhou attacks & spell cards after each goal.

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You are a regular human, living in today's world, trying to go to work, however due to some strange circumstances, you suddenly find yourself in Gensokyo, unable to go back, and caught in the middle of an incident caused by someone who seemingly appeared in Gensokyo out of nowhere, and has ruined the balance.

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Touhou games & fangames usually have great music, and this game is no exception, you have goofy songs like "Palaço!" & "Chuchu the Choo-Choo Train", and nice jazzy & swing songs like "Ace and Queen" & "Monkey King Swing".
A lot of the songs fit into the genre of Jazz & Swing, being less or more intense depending on the situation, and I think it fits the game really well, it especially fits Kakkoi & everything surrounding her. The game also has more intense, "epic boss fight" songs like "Boundless" and they sound fantastic too. In general, the soundtrack is great, it fits the game, and it's catchy, I especially like Kakkoi's themes, "Kakkoi" and "Cosa Nostra".

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⦁ I loved this game from start to finish, a lot of the jokes are executed perfectly, to the point where I found myself with already 40+ screenshots within the first 50 minutes of me playing the game.
⦁ I encountered some small bugs here and there, nothing too problematic, such as some characters having other characters' voices, but aside from that, everything went smoothly on my end.
⦁ The air hockey gameplay was surprisingly very fun, and as the game goes on, you're introduced to attacks that are more challenging to dodge while still being fair, it's a learning experience, and there's a trick or pattern to most of them, just like regular Touhou games.
⦁ I am a very huge fan of Kakkoi Elbertson, the original character in this game. She has some Touhou elements to her design while having some funny creative liberties like the sunglasses, the gun, and the cigar. I love every line of dialogue from her, since they always have the most creative, elaborate comebacks and her very laid-back attitude and frequent use of slang is a funny contrast compared to the characters who interact with her.
Personality wise and story wise, I also really like her, she has authority, she's tough, she has no filter and says whatever she thinks, and despite the attitude, lack of filter and constant roasting you realize through some of her dialogue that she's actually a nice and interesting person, one line I really liked from her that shows her values & personality beyond what you see from the main dialogue is when she criticizes Satori's decision to fire Okuu, saying that even though what she did to get fired was a big deal, Okuu was very loyal, and loyalty is an important quality, you don't just squander it like that.
If she was an official Touhou character, she'd have taken the #1 spot in my favorites no questions asked. Some Touhou OCs I've seen in games have neat designs, but end up being very unremarkable, and don't hold a candle to official counterparts if they have any, but Kakkoi is definitely unique in a very good way.
⦁ Overall, I've already said this, but it's a fantastic game, I definitely recommend it, and I don't mind going back to 100% it, or play some more blackjack with Kakkoi.