There is no directway or visual interface for configuring Gravity Forms in order integrated with other 3rd party API such as ActiveCampaign. So, the only way is by modifying or hooking Gravity Form submission on Wordpress. We should modify/hook gform_after_submission function so it will be POST to ActiveCampaign API as well.
But, we could still provide the list for ActiveCampaign email so admin can configure it.
If you agree with this method, then I could provide the quick solution for this.