Ok, so I finally noticed (duh!) that when running the game from the GM IDE, it is listening on a non-standard port (51268 for example). Since I didnt openthat listen port on my Apache server, I finally realized that the IDE launches it's own "web server", which listens on the specified port. Even though I have set GM:S to know where my web root is, it doesnt read the crossdomain.xml file in the web root folder.
So, here is how I got it to run:
1. put a valid crossdomain.xml file in my web root. My file is simplistic:
<?xml version="1.0" ?> <cross-domain-policy> <allow-access-from domain="*" /> </cross-domain-policy>
2. Then, I used the Create Application function of GM:S, and specified a new folder under web root (so I wouldn't overwrite the files I use for in-IDE testing). For example: d:\wamp\www\gamename\build\
GM:S then faithfully compiled the app to this folder.
3. Open the app in my browser by using the proper URL (ex: http://my.domain.com/gamename/build/) and it works just fine.
Hope this helps out the OP and whoever else needs it!