Gravity Forms HTML5 - custom validation message support (https://wordpress.org/support/topic/custom-validation-message-support-2/)

custom-validation-message.diff

diff --git a/gravityforms-html5-validation.php b/gravityforms-html5-validation.php
index 5e2c9e0..116714c 100644
--- a/gravityforms-html5-validation.php
+++ b/gravityforms-html5-validation.php
@@ -599,6 +599,12 @@ public function gform_field_content( $field_content, $field, $force_frontend_lab
 				break;
 		}
 
+		if ( $field->validation_message ) {
+			if ( $element = (( $result = $xpath->query( "//input" )) ? $result->item( 0 ) : null ) ) {
+				$element->setAttribute( 'data-validation-message', $field->validation_message );
+			}
+		}
+
 		$field_content = $doc->saveHTML();
 
 		// Remove our html wrapper from processed field.
diff --git a/js/gravityforms-html5-validation.js b/js/gravityforms-html5-validation.js
index 6e66dab..917ef7c 100644
--- a/js/gravityforms-html5-validation.js
+++ b/js/gravityforms-html5-validation.js
@@ -20,6 +20,7 @@             function( elem, i, match ) {
     var debug = window.GFHtml5Validation.Debug = false;
     var pageValidating = false;
     var pageValid = false;
+    var validationMessage = false;
 
     var trace = function( message, target ) {
         if ( debug ) {
@@ -40,6 +41,9 @@  function( message, target ) {
         // Validate current page inputs
         $("[name^='input_']:not([type='hidden'])", $currentPage).each( function() {
             pageValid = this.checkValidity === undefined || this.checkValidity();
+            if ((validationMessage = this.data('validation-message'))) {
+                this.setCustomValidity(pageValid ? '' : validationMessage);
+            }
             return pageValid;
         });