Coldfusion error http://localhost:8500/flex2gateway/ bugs the hell out of me every now and then.
Generally, it means you have an issue with your services-config.xml file in your "WEB-INF/Flex" root (and any subsequent include files, as in remoting-config.xml, messaging-config.xml, etc.
After googling the hell out of it, the final solution was to look at the cfserver.log (which lives in Coldfusion8/logs) and hand fix the xml until it's bug free.
- broken xml structure (i.e. end tags missing)
- using the same endpoint for two different services, which CF8 isn't a happy camper about (i.e.
- missing channels for a service