In Shotokan Karate when do you learn Gojushiho Dai?

Gojushiho is considered an highly advance kata. And is required around your 4th or 5th dan ranking, Depending upon the organization.

When you learn the kata is a little more difficult to answer most of the time instructors teach kata in order, but there is a point in your training, when you learn kata when you are ready to learn them. Although not all instructors prescribe to this practice.

Then you have other instructors that just teach students kata, because they chase kata and want to learn as many as they can.

It all depends upon your instructor when you learn the kata.

its possible

Sure...