To do this you would need use a field that is not attached to ACF in GF. The you would need to use the a gform_after_create_post action to get the values submitted by GF and then use update_field() to insert the value into the gallery field.
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The most recent ACF Chat Friday featured a live demo of how to register CPTs directly in the plugin, one of our most requested features. Check out the summary below for a replay of the demo, and don’t forget to register for the next session! https://t.co/k2KQ3WWBAz