You can have any MemberGate generated page or product on your site attached to a affiliate code.
For example If the affiliate code was "privatead" the code would be - ?affID=privatead
Just append the affiliate code at the end of the URL on your own site.
An example would be:
or, if you are using MemberGate version 7 or higher, and have the use of Keyword Rich URLs, you affiliate link would look like this: