Without testing, this should work (still use the add_filter bit):
function cwwp_custom_gforms_spinner( $src ) {
global $post;
if( $post->ID == $id ): // use whatever page identifier/conditional you like here: ID, template used, slug etc.
// other identifiers would probably be better and won't rely on global $post
// for example is_front_page(), is_archive(), etc
return get_stylesheet_directory_uri() . 'path/to/spinner/1.gif';
else:
return get_stylesheet_directory_uri() . 'path/to/spinner/2.gif';
endif;
}