Randomly choose a gateway from valid gateways.

pmpro_valid_gateways.php

<?php

// Add "authorizenet" to valid gateways.
function my_pmpro_valid_gateways( $gateways ) {
	global $gateway, $pmpro_valid_gateways;
	$gateways[] = 'authorizenet';
	$pmpro_valid_gateways = $gateways;
	return $gateways;
}
add_filter( 'pmpro_valid_gateways', 'my_pmpro_valid_gateways' );

// Randomly choose a gateway from valid gateways.
function my_pmpro_checkout_preheader_before_get_level_at_checkout() {
	global $gateway, $pmpro_valid_gateways;
	$gateway = $pmpro_valid_gateways[rand( 0, count( $pmpro_valid_gateways ) - 1 ) ];
}
add_action( 'pmpro_checkout_preheader_before_get_level_at_checkout', 'my_pmpro_checkout_preheader_before_get_level_at_checkout' );