Gravity Forms readonly fields (no JavaScript).

functions.php

<?php

/**
 * Read only
 * 
 * @param string $field_content
 * @param array $field
 * @return string
 */
function prefix_gform_field_readonly( $field_content, $field ) {
	if ( isset( $field['cssClass'] ) && false !== strpos( $field['cssClass'], 'readonly' ) ) {
		$attr = 'readonly="readonly"';

		if ( false === strpos( $field_content, $attr ) ) {
			$search = array(
				'<input ',
				'<textarea ',
				'<select ',
			);
			
			$replace = array(
				"<input $attr ",
				"<textarea $attr ",
				"<select $attr ",
			);
			
			$field_content = str_replace( $search, $replace, $field_content );
		}
	}
	
	return $field_content;
}

add_filter( 'gform_field_content', 'prefix_gform_field_readonly', 10, 2 );