Limit length of first name field in Gravity Forms. Applies to ALL FORMS.

functions.php

add_filter( 'gform_field_validation', 'kuztek_first_name_validation', 10, 4 );
function kuztek_first_name_validation( $result, $value, $form, $field ) {
    $first_name_max_length = 10;
    $first_name  = rgar( $value, $field->id . '.3' );
    if ( $result['is_valid'] && 'name' === $field->type && strlen( $first_name ) > $first_name_max_length ) {
        $result['is_valid'] = false;
        $result['message'] = 'Please enter a first shorter name';
    }
    return $result;
}