I just started gdisk with the respective disk as a parameter. It recognizes automatically that the disk has a DOS MBR and proposed to convert it to GPT. I just need to write the GPT with "w".
That's it. I made a copy of the MBR before with "dd if=/respective/disk of=backup.file bs=512 count=1", and also of the first 2048 sectors with
"dd if=/respective/disk of=backup_first_2048_sectors.dd bs=1048576 count=1".

Bernd