You can play ISO using an ISO loader if your Go is on firmware 6.20 or 6.35 (6.20 is best for homebrew/iso loading), You need to use HEN (homebrew enabler) for those respective firmwares.
HEN will make your PSP act like it is running custom firmware, but when the system is hard reset it will revert back to the original firmware.
Some homebrew can be launched on 6.37 (the latest firmware as of this post) but not a lot and definitely no ISO loading. Do not upgrade to 6.37 if you value homebrew/iso loading.