I have at the moment a few partitions with windows, KaOS and a few test partitions with a few versions what don't work when I boot this.

I will delete this partions and that I don't see this when I boot my computer.

How to do this?

If you're working from the partition as where grub is installed, and if you really want to delete the partitions of the hard disk do it with kde partition manager or gparted and then updates the grub

sudo grub-mkconfig -o /boot/grub/grub.cfg

make sure that partitions to delete and not delete the partition where grub is installed.

If you need re-install grub:

I use here gummiboot. Is this the same as grub?

For UEFI, to remove entries from the menu you use efibootmgr. Adjust the title of your post to show it has nothing to with GRUB. This is for systemd-boot.

Start with efibootmgr -vit will show all entries with needed numbers and id.

To remove one (as root) efibootmgr -b 0003 -B where 0003 is the hexadecimal number representing the entry you want to delete. Repeat for any others you want to remove, always checking with efibootmgr -v after every deletion what the numbers are.

This is the output

[fred@fred-pc ~]$ efibootmgr -v

BootCurrent: 0005

Timeout: 1 seconds

BootOrder: 0005,0001,0002,0003,0000

Boot0000* ubuntu HD(2,GPT,ff0b5c90-c0db-430c-8ba5-3e828594656f,0x190800,0x82000)/File(\EFI\ubuntu\shimx64.efi)

Boot0001* antergos_grub HD(2,GPT,ff0b5c90-c0db-430c-8ba5-3e828594656f,0x190800,0x82000)/File(\EFI\antergos_grub\grubx64.efi)

Boot0002* Windows Boot Manager HD(2,GPT,ff0b5c90-c0db-430c-8ba5-3e828594656f,0x190800,0x82000)/File(\EFI\Microsoft\Boot\bootmgfw.efi)WINDOWS.........x...B.C.D.O.B.J.E.C.T.=.{.9.d.e.a.8.6.2.c.-.5.c.d.d.-.4.e.7.0.-.a.c.c.1.-.f.3.2.b.3.4.4.d.4.7.9.5.}...9................

Boot0003* ubuntu HD(2,GPT,ff0b5c90-c0db-430c-8ba5-3e828594656f,0x190800,0x82000)/File(\EFI\Ubuntu\grubx64.efi)

Boot0005* Linux Boot Manager HD(2,GPT,ff0b5c90-c0db-430c-8ba5-3e828594656f,0x190800,0x82000)/File(\EFI\systemd\systemd-bootx64.efi)

I can delete here what you see above:

Boot0000*, Boot0001 and Boot0003*

Boot0002 I need for windows and Boot0005 is the boot manager I now use.

Is this correct what I think?

Yes that sounds correct, plus if you remove too much and a disk has an UEFI boot partition, your UEFI BIOS will create an entry anyway.

And to explain a bit more about the simplicity of using systemd-boot over GRUB. Once you remove an install for which you added a <os_name>.conf (like last year with kaos-kf5.conf) in /boot/loader/entries, all needed is to just remove that file, no need to run any command or edit any lines, entry is then automatically removed from your (KaOS) bootloader menu.

Thank you I have delete the old bootloaders and the .conf I don't use anymore