HOW TO USE GRAVITY FORMS WITH MEMBERSHIP 2 - AND MORE

Am afraid I posted this also in the Forum section (by mistake).

I have a membership site where people first have to be invited to join and can register as users (Subscribers). After approval they can then purchase a membership with Membership 2 Pro. They can only do so after having logged in.

1) I need to use a Gravity form for Users to purchase a membership. Users can apply a discount if they are students and this is currently not possible, as the PayPal payment form appears immediately when logged in users want to subscribe for or renew their membership. How do I get Membership 2 Pro to recognize/use Gravity forms using the PayPal gateway?
2) I can't find the area where I was able to define Invitation codes. They were in Membership > Invitation Codes, but are no longer there.

PLEASE NOTE: I had to turn off the WPMU Dashboard, as it caused serious conflicts with this site, specifically with the login routine for members.