Well, how can I change the username in GOG?

Hello.

If you want to change your username, you'll have to contact support.

Go here:
https://support.gog.com/hc/en-us/requests/new

Select "Account and Website" as the issue type and "account username (nickname) change" as the problem type. Then, fill in the rest of the fields (e-mail address, subject and description) and you should be good to go.

Thanks