Use Gravity Forms to Register User and Add to Membership Level?

Hello, I want to offer a user registration with membership registration options on the sales page.

For example at the end of the page, there is a user registration form with 2 radio buttons.
- 7-day trial $17.99
- Monthly Membership: $34.99

The user will put in his desired username, password, and email; select his option, and click buy.

I can't find a way to do that with M2; but I THINK you can do it with Gravity Forms.

I want to use Gravity forms to do it. I have the Gravity Forms user registration plugin, and the stripe payment plugiun, but how can I assign a user to a membership level?