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<?php
/**
 * GP Multi-page Navigation // Gravity Perks // Disable Navigation After Set Page
 */

// update the "673" to your form ID
add_filter( 'gform_pre_render_673', function( $form ) {

	$submission = rgar( GFFormDisplay::$submission, $form['id'] );

	// if submission data is not set for this form, we'll assume we're on the first page
	if( GFFormDisplay::get_current_page( $form['id'] ) >= 3 ) {
		$form['pagination']['gp-multi-page-navigation_enable'] = false;
	}

	return $form;
} );