Geminis are caring and eager to learn. However, they can be flighty and anxious. Glad to share what they know, Gemini make excellent lawyers and innovators. They are fascinated by how the world itself works and learn quickly. Gemini wouldn't do well in routine jobs that force them to complete mundane tasks over and over again.
Geminis are extremely sociable and hate to be alone. They have extremely open minds, as well as the desire to explore, and therefore make excellent writers and members of sports teams. And in that same vein, here are five Hogwarts subjects Gemini would excel in, and five they would fail at.
Excel: Care of Magical Creatures
One of Gemini's main traits is their gentleness. Therefore, they would more than likely excel at Care of Magical Creatures, which is focused on learning about the creatures that inhabit magical Britain and beyond.
Geminis also like to learn everything they can about a subject that interests them and share it with others, which would definitely fit with appreciating animals. Working with animals also ensures that a Gemini wouldn't be alone.
Fail: Muggle Studies
Muggle Studies, as shown by the books, is a class that is taken mainly because it is easy, and not because wizards think they will be working with many muggles.
Therefore, most Geminis probably wouldn't enjoy it very much, especially as much of the information is interesting, but not relevant or important to a future career. It also seems to be a small class.
Excel: Quidditch
Geminis are generally sought-after for team sports, and would naturally love Quidditch. Their sign belongs to the element of air and possess natural flexibility.
Given their habit of sharing everything they learn with others they meet, it would be fitting to be able to share information with teammates from the Quidditch team. After so much book learning, it's also helpful to get outside and feel the wind in one's face.
Fail: Gobstones
Gobstones is a wizarding game that is essentially magical marbles. There isn't that much strategy involved, and it's mainly seen as a game for children.
Therefore, it isn't mentioned in the Harry Potter series very much and mainly exists to fill out the world. Given the Wizarding World's emphasis on Quidditch, Gobstones isn't given that much attention. However, though a Gemini might be interested in reading about it, they wouldn't get much enjoyment out of playing it.
Excel: Potions
Potions is a subject Gemini would excel at due to its necessity for precision and finesse. Gemini, interested in how the world works, would latch onto Potions and would seek to figure out how ingredients relate to one another.
It's easy to see how they would become innovators in the field and seek to improve and expand on potions people have used for decades. This discipline allows Potioneers to both learn theory and harness their own creativity to help improve people's lives. It's also a class where people can pair up and solve problems as a team.
Fail: History of Magic
Given that History of Magic is taught by a boring ghost (Professor Binns) it's hard to see how people who are so given to self-study would find anything interesting in this class.
History can also be learned through reading about it, as most Geminis would do. It also doesn't seem as though the Wizarding World has that many historians, which would make it a necessary but not necessarily important class.
Excel: Transfiguration
On the other hand, Transfiguration is a constantly-changing field that Geminis would want to be a part of shaping or improving.
Therefore, this is a discipline that many Geminis would want to succeed at, due to its usefulness in most jobs. Transfiguration also combines personal creativity with theory that can only be learned from books. Ergo, most Geminis would appreciate this field.
Fail: Defence Against the Dark Arts
Defence Against the Dark Arts is a very individualistic discipline, which wouldn't fit with team-based Gemini very well. Defense also constantly requires quick thinking and working creatively.
Though there is theory involved, for the most part, Defence requires inspirational solutions to the problems it poses. Though a Gemini could read all about different types of dark creatures, if they hadn't faced each one individually and truly performed those spells, it's unlikely they'd truly understand the subject.
Excel: Astronomy
Mapping the stars is something interesting, but that doesn't lend itself very well to a magical career. However, it is a theory-heavy subject as shown by canon, and is, therefore, something many Geminis would succeed at.
There's something very calming about studying the stars and the sky, and it's easy to imagine Gemini getting caught up in the excitement of unknown discoveries.
Fail: Divination
Divination is a discipline that is either understood or it isn't. There isn't really any way to learn the true Sight, though other disciplines such as palmistry can be learned from books. Therefore, it isn't a class that many Geminis would enjoy.
Gemini will generally excel at theory-heavy classes that also allow them to be creative, such as Potions or Transfiguration, so a class that is neither here nor there like Divination is less likely to be understood.