Specify an entry image or a default image to the og:image Open Graph tag.

my_gv_wpseo_opengraph_image.php

<?php
add_action( 'wpseo_opengraph', 'my_gv_wpseo_opengraph_image', 10, 1);
function my_gv_wpseo_opengraph_image( $img ) {
    if( ! function_exists('gravityview_is_single_entry') || ! function_exists( 'gravityview_get_entry') ) {
        return $img;
    }
    // get single entry id
    $entry_id = gravityview_is_single_entry();
    if( empty( $entry_id ) ) {
    	return $img;
    }
    $entry = gravityview_get_entry( $entry_id );
    $form_id = empty( $entry['form_id'] ) ? '' : $entry['form_id'];
	switch ( $form_id ) {
		// Customise to your own case FORM ID
		case '30':
			// Customise to your own FIELD ID
			$img_field = '1';
			break;
	}
    if( isset( $entry[ $img_field ] ) && !empty( $entry[ $img_field ])) {
    	$img = $entry[ $img_field ];
    }else{
	        //specify here a default image
		$img = 'http://yourwebsite.com/wp-content/uploads/2017/07/Image2.jpg';
    }
    $GLOBALS['wpseo_og']->image_output( $img );
}