Contact gportal support. If they can't do it for you, they will tell you how.

The character is on your end, gportal should be able to reset the world to a new seed.
You can have as many characters as you like, just create a new one.

Maybe you could start a world on your own computer, then upload it to GPortal... here is the cut and paste:

How to upload your own world on your Valheim server

Start and join your server once (This creates the savegame folder structure, this is a REALLY important step!)
Stop your server (otherwise you cant upload a savegame!)
Locate the savegame on your computer. It is usually located here: C:\Users\username\AppData\LocalLow\IronGate\Valheim
There you need the .db and the .fwl file.
Connect to your server, using the FTP Connection
Upload both files (.db and .fwl) to this folder: .config\unity3d\IronGate\Valheim\worlds (or save\worlds, this is the same folder, just with a different path!)
Now head to the basic settings again. The server still should be stopped completely.
Rename the worldname from (default) “world” to the name of the files you uploaded (no file extension, just the name).
AFTER the previous step, start the server and join it.
If everything works, you should think about doing a backup.

Source: https://www.g-portal.com/wiki/en/how-to-upload-your-own-world-on-your-valheim-server/