Yes, you are attempting to use the field key parameter which only means anything to ACF itself, it doesn’t mean anything to the actual connection between Gravity Forms and the Custom Field property. That’s what the field name is before. It’s usually the lowercase, underscored version of your field title. “Business phone” becomes business_phone usually.
That’s what you need to choose from the dropdown, or if it doesn’t exist yet, just enter the field name under ‘New’ and as soon as GF saves it, it will switch to “Existing” with the field name in the dropdown.