1) Use a repeater field, create an acf/save_post action that copies the images from the repeater to a gallery. This would be my choice because it’s more secure.
2) Give contributors access to the WP media library. There may be a plugin that can do this. Here’s one that I’ve never tried https://wordpress.org/plugins/members/. Either you’ll need a plugin that allows it or you’ll need to start searching for code to do the same thing.
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