Hi,
I have written Payment Gateway for different payment processors, like GoogleWallet, PaymentWall, 2Checkout, AmazonFPS etc (For vBulletin and wordpress) and can write for Stripe too. If you have already 1 gateway "PayPal" set there it will be easy to add more.
For my experience and expertise, please see my feedback.
Thanks.
Regards,
Pankaj Shrivastava