Yes. The GUI adds a checksum to the 'naked' address, I think the first nine trytes of the address hash, that's why they are not 81, but 90 chars long in the GUI
Thanks, the valid format is 81 or 90 chars, uppercase letters and 9's. Is that right?
Yes, 81 chars if sending from console or nostalgia, and 90 chars if sending from GUI.
If you generated a 81 char address previously: You can also calculate the checksum in the GUI and append it to a 81 char address, to make it 90 char long.
Why are you asking?