After installing the GSX its time to input the key file again... now the pain comes!!!
I tried to use the regedit, and give me a error...
The problem doesn't seem to have anything to do with GSX, but with REGEDIT. You don't say which error you get, so it's not easy to help you.
How can i input the key on my GSX, to use again?
The .REG file is not a "special" GSX file. It's the standard format used by REGEDIT to store a registry key so, normally, it should be enough to double-click on it, and accept to insert the key in the registry.
However, the .REG file is not really needed or used by GSX, it's just a convenience to saving you from MANUALLY typing your Serial Number, risking mis-typing it, since it's ensure whatever key is in there, is correct, since it was created automatically so, there's no risk mis-typing it.
But of course, if you have a problem with REGEDIT, usually caused by antivirus that try to block access to it, you can just type the Serial Number manually, using the "Register Serial" button on the Addon Manager menu.