This add-on allows you to create forms which directly create and subscribe users in AcyMailing.
The first step is to create a new form on Gravity Forms, in the advanced fields insert an email field and also the field AcyMailing List:
In the field AcyMailing Lists you have 4 settings:
Field Label: the title that will be displayed before the field
Displayed lists: the lists displayed with checkboxes
Lists checked by default: the lists that will be already checked when the form is displayed
Automatically subscribe to: the lists which the user will be automatically subscribed