Mh, maybe it is something how Gravity forms fills out the post_thumbnail field and/or saves posts and custom fields? I managed to fill other (text-based) fields with Gravity forms frontend posting, but so far the image field fails (had a similar problem with a date picker field – even though it had the same format, it never reached the actual ACF field when saving a post with Gravity forms). So maybe the complication is somewhere along GF’s creation of posts.
