Upgrade OSX GM to final version I want to install OSX Yosemite GM, but final version will release soon, can I upgrade Yosemite GM to final through Appstore when apple release final version?
or I have to download another 5GB ?
You don't need to.

Apple is regarding the last GM and last public beta as the final release of Yosemite and not offering an update path. Instead, wait for 10.10.1 and the Mac App Store will include this as an update path putting you back on track.