Change wp_get_attachment_image attributes (src and srcset) on specific custom field

You won’t be really able to tell which context the image is being used in in the wp_get_attachment_image_attributes filter, but you can pass custom attributes to wp_get_attachment_image() in the template directly with the 4th argument. Then you can use wp_get_attachment_image_url() and wp_get_attachment_image_srcset() to get the values for those attributes:

$image = get_field('image');
$size="gallery";

echo wp_get_attachment_image( $image, $size, false, array(
    'src' => '',
    'srcset' => '',
    'data-flickity-lazyload-src' => wp_get_attachment_image_url( $image, $size ),
    'data-flickity-lazyload-srcset' => wp_get_attachment_image_srcset( $image, $size ),
) );