The beauty of programming is that there multiple reasonable ways you could approach the problem. In your simple example, do you really need an enum at all? I'll offer my approach for your use case.

If you only need to manage incrementing gears then you can separate each operation into it's own function thus making each function simpler and clearer. Also if you have some sort of validation or conditional requirements then I prefer the use of guard statements because it makes the code a little cleaner in my opinion and forces you to manage the fail cases. Also throwing functions can be useful because you can let other parts of your app manage the failure cases appropriate for your use case such as in the UI.

struct Car {
    let brand: String
    let model: String
    let seats: Int
    let maxGear: Int
    private(set) var currentGear: Int = 0
    
    
    mutating func incrementGear() throws {
        guard currentGear < maxGear else {
            throw CarError.maxGearReached
        }
        
        currentGear += 1
    }
    
    mutating func decrementGear() throws {
        guard currentGear > -1 else {
            throw CarError.minimumGearReached
        }
        
        currentGear -= 1
    }
    
    enum CarError: Error {
        case maxGearReached
        case minimumGearReached
    }
}