according to the dev team:
When you click the "iPwner" button in PwnageTool, your main s5l8900 bootloader gets pwned. To undo this, use iTunes to restore to a Apple ipsw.
When you neuter using BootNeuter, your S-Gold radio bootloader gets "pwned". To undo this, run BootNeuter again and turn off all options (and pick 3.9 or 4.6 depending on your preference).
Two different CPUs, two different tools. But both the s5l8900 pwnage and S-Gold pwnage are 100% reversible.
If you want to relock your phone - use bootneuter and click lock to off. Then restore with Apple firmware. And you are back to factory fresh.
If you restore with Apple firmware you will then have to use ipwner again to use custom firmware.