plugin install

wp/wp-content/plugins/gravityforms/includes/settings/js/select-custom.js

@@ -0,0 +1,45 @@
+window.addEventListener( 'load' , function() {
+
+	var $selectOptions = document.querySelectorAll( '.gform-settings-field__select_custom select' );
+	var $buttons       = document.querySelectorAll( '.gform-settings-select-custom__reset' );
+
+	$selectOptions.forEach( function( $select ) {
+
+		var $inputField = $select.parentNode.nextSibling;
+
+		$select.addEventListener( 'change', function( e ) {
+
+			if ( e.target.value !== 'gf_custom' ) {
+				return;
+			}
+
+			// Hide drop down, show input.
+			$select.style.display     = 'none';
+			$inputField.style.display = 'block';
+
+		} );
+
+	} );
+
+	$buttons.forEach( function( $button ) {
+
+		var $inputField = $button.parentNode;
+
+		$button.addEventListener( 'click', function( e ) {
+
+			// Hide input, show drop down.
+			$inputField.style.display = 'none';
+			jQuery( this )
+				.closest('div.gform-settings-field__select_custom')
+				.find( 'select' )
+				.each( function( index, element ) {
+					element.value         = '';
+					element.style.display = 'block';
+				}
+			);
+
+		} );
+
+	} );
+
+} );