mypmpro-element-class-generatepress.php

<?php
/**
 * This recipe will add in the necessary classes to PMPro Buttons, ensuring they
 * have the same look & feel as your theme: GeneratePress
 *
 * https://wordpress.org/themes/generatepress/
 * 
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function mypmpro_generatepress_classes( $class, $element ){

	if ( in_array( 'pmpro_btn', $class ) ) {
		$class = array();
		$class[] = 'button';
	}

	return $class;

}
add_action( 'pmpro_element_class', 'mypmpro_generatepress_classes', 10, 2 );

This recipe is included in the blog post on "Using the GeneratePress Theme by Tom with Paid Memberships Pro" at Paid Memberships Pro here: https://www.paidmembershipspro.com/generatepress-theme-by-tom-paid-memberships-pro/