Customise your gravity form submit function by adding this code to your functions php file.

add_class_to_submit_btn.php

add_filter( 'gform_submit_button', 'add_custom_css_classes', 10, 2 );
function add_custom_css_classes( $button, $form ) {
   $dom = new DOMDocument();
   $dom->loadHTML( $button );
   $input = $dom->getElementsByTagName( 'input' )->item(0);
   $classes = $input->getAttribute( 'class' );
   $classes .= " btn btn--primary";
   $input->setAttribute( 'class', $classes );
   return $dom->saveHtml( $input );
}