Modifies the list of valid gateways used by PMPro. By default, it returns the gateway set in Memberships > Payment Settings in the WordPress Dashboard.

apply_filters( "pmpro_valid_gateways", array( "paypal", "paypalexpress" ) );

Parameters

array (pmpro_getOption(“gateway”, true))

Source

File: https://github.com/strangerstudios/paid-memberships-pro/blob/master/preheaders/checkout.php