Bad idea and here is why:
GM will expire. Despite what people claim it does not count as the final version and here is exactly what will happen if you do this and don't upgrade to the regular version (checking in iTunes will tell you its the latest version but it will expire).
You download GM, it explores. You try and upgrade/restore in iTunes and you can't, because iTunes is confused (unless Apple fixed this issue) and thinks you have a current version.
You then have to do a backup and then a DFU restore and restore using the IPSW file and then restore everything back on your phone. It's a giant pain in the ass and takes awhile.
*SIGH* The GM's don't expire, it is the same software as on the iPhone 6 you're getting today. The GM don't even require a developer account to install. There will NOT be some magical different software on the iPhone 6. All major telecoms in my country warned NOT to install iOS 8 if you use call forwarding, cause that is broken. It will be fixed 8.0.1, but they didn't put a date on that update.