You can fork ActiveMerchant on github at
http://github.com/Shopify/active_merchant. You can develop your
gateway in your fork and then send a github pull request to us when
you're all finished. Once your gateway is ready and reviewed it can be
merged into ActiveMerchant.
--
Cody Fauser
http://shopify.com - e-commerce done right
http://www.codyfauser.com - blog
http://peepcode.com/products/activemerchant-pdf - ActiveMerchant PeepCode
http://www.oreilly.com/catalog/rjsrails - RJS Templates for Rails