yes, game decrypter is required to play GT (PSP version) on M33. chillax man :D.
well the easyest way is to downlaod 5.50 gen.
6.10 gen is out on the 20-24 december 2010 if you wait till ten its half the hassle
put the 550.pbp on the root of your memory stick then put the update file in psp\game
after that run the update
leave your psp while it is putting the cfw on the psp. if you dont and it formats all the flash parts it will brick your psp and you wont even be able to access the recovery menu and then you will have to buy a pandora battery
i have bricked my psp befor but luckly i have 2 cfw psp's and i just made another pandora battery.
after you have installed the the cfw you are done but you may want some homebrew apps plgins ect.