Ok folks, the problem is Gravity Forms, not ACF. What you have to do is make the Gravity Form upload field point to a basic ACF TEXT field.
When you upload something through GF, it passes a URL to the location of the uploaded file. ACF needs more than a URL to recognize it as media. So unless GF releases an update where it returns an image object rather than a URL, this is the only work-around available when trying to use these plugins together.
