<?php echo S2MEMBER_CURRENT_USER_SUBSCR_GATEWAY; ?>
This may output something like: paypal
(this is the Paid Subscr. Gateway associated with their Paid Subscr. ID)
(possible values: paypal, alipay, authnet, ccbill, clickbank, google)

---- s2member Shortcode Equivalent ----

[s2Get constant="S2MEMBER_CURRENT_USER_SUBSCR_GATEWAY" /]