Then, use conditional logic based on that choice, to show a product with the new pricing. Then, you can set up your payment feeds based on that radio button choice as well, in order to set the duration of the subscription and the price of each payment. If you need more assistance with that, I recommend opening a support ticket:
Thank you Chirs for support,
I try to explain myself better
The discount must be calculated on the total (not on a single product). I list several items of services to buy that change the grand total.
After these services can be chosen for the various months, and here the discount takes place: 3/6/9 months etc.
You can use the same approach I detailed. You are not really applying a discount using the method I showed: you are conditionally showing a different product, which has a discounted (different) price. Does that make sense?
There are no conditional pricing capabilities in Gravity Forms. There is a 3rd party plugin that can help, but I think the method I outlined could work as well.