The Golden Master version means that it is the shipping version of the software that developers must use to develop and submit iPhone apps that are compatible with iOS 4.0.
iPhone Dev Team had recently released Redsnow 0.9.5 Beta 4 to jailbreak iPhone OS 4.0 Beta 4. The beta version of RedSn0w is targeted only at the jailbreak app developers and is not meant for regular iPhone users. The Dev Team had noted that the beta release allows jailbroken app developers to fix "broken" pieces of code in their applications ahead of the public release of iOS 4.0.
For those who can legitimately activate to the iPhone3G 4.0GM seed, the “point to previous IPSW” redsn0w trick works and will jailbreak your 4.0GM. The “trick” is to point redsn0w at the 4.0beta4 IPSW instead of 4.0GM (do this after you've updated to 4.0GM and activated through the new iTunes 9.2 on your Mac). Since the carrier unlocks aren't yet supported at 4.0, don't do this if you use yellowsn0w/ultrasn0w/blacksn0w.
For those without legit iPhone3G 4.0GM seed access, please wait until the full public 4.0 release for updates to the jailbreak tools.
Apple has announced that iOS 4 will be released to users on June 21, so Dev team should be releasing tools after June 21. It will be interesting to see if the Dev team can also jailbreak iPhone 4 that was unveiled yesterday.
[via Dev Team]