This step-by-step procedure will show you how to make a DMG package from the GNS3 source code. I’ve built the package on Mac OS X Snow Leopard and used Py2app to make a Mac OS X application. Also, have a look at this article that really helped me.
This procedure worked for me, please let me know if you encounter any issue and I’ll update it. Also, please tell me if you can make it work on Mac OS X Leopard (10.5.x).