Hi Rexa76,
To convert your Drive to GPT:
At the very start of the Installation Process, press Shift + F10, or Shift + Fn + F10 to open Command Prompt
Run this command and hit Enter:
diskpart
Run this command and hit Enter:
list disk
(Make note of the disk number you want to convert and enter it into the next command - replace disknumber)
Run this command and hit Enter:
select disk disknumber
Run this command and hit Enter:
clean
Run this command and hit Enter:
convert gpt
Then close Command Prompt