Need to change default Membership gateway!

I am using membership and have the membership access levels and subscriptions set and working. However, I had to create my own registration form using Gravity Forms so that there is conditional logic for age verification purposes. I could put a "I am over 18" checkbox on the Membership sign up form, but I dont want that showing up on their profile. When a member signs up they are assigned the correct new member level, however there is no upgrade available using the PayPal single payments because they werent assigned that gateway at sign up. I could create a Paypal subscription for Gravity Forms, but it would not be recognized by Membership plugin and I would have to change the level manually. Is there anyway to just default the gateway to Paypal single payments so that they are assigned Free level but still have upgrade path?