I may be interested in developing this, I have already completed three other payment gateway modules and published them on drupal.org.

http://www.goemerchant.com/techsupport/gateway.htm lists a few different options, depending on whether you want to take credit card details on your own site or pass users over to GoEmerchant's own pages. The EZ Payment Gateway looks the simplest and should only take a few hours to implement and test, the XML gateway would take slightly longer.

Please get in touch via private message if you would like me to quote on this work.