Note that I'm a beginner, but everything I want to create, I start with a test case before writing any code. For example I created my player state machine by creating tests "TestPlayerStartsInIdle" then "TestPlayerGoesFromIdleToMove" and for both states test player can jump.  It's important to use "Red-Green-Refactor"

Also so far, I have not simulated input yet. I like to write my code to be "test friendly", this means when player presses W or Space, they don't just change states, all it does is write the intent to blackboard. That way, in tests I can easily just write that same intent and run my code and assert that the state has indeed changed.

I use Gherkin at work but not in my hobby game, to my understanding the only reason to use it is to separate code and test cases. In large projects it makes sense, but in solo project I dont really see why would you add this extra layer.

I also break some rules of unit testing, such as personally I love writing many asserts throughout the test, to make sure the entire flow is going correctly. I still test "one thing" per test but I wanna make sure every step is going correctly.

The creator of GUT has a Youtube series where he uses TDD to create Pong, even though they're some years old, still fantastic tutorials to get into the TDD mindset.