Polyphony Digital has announced that Gran Turismo Sport will be getting a closed beta, that begins later this week: 17th March 2017. You can apply to take part in the beta, but as places are limited, only a small selection of applicants will get access to the beta.
You will need a PSN ID to apply to take part, and the sign-up pages for each region are different:
In the beta the cars and tracks will be regularly rotated so players get at shot at testing a variety, which in turn will provide better feedback to the developers. The beta will also only be open at certain times so the development team in Japan can implement changes quicker from the feedback. There is a mention of the beta eventually rolling out to more of the community, but details regarding when that will happen are to be confirmed.