Convert from MBR to GPT WITHOUT DATA LOSS and NO WINDOWS

From within any Linux system (including Gparted Live), open a terminal and run gdisk on the device representing your disk. For example, if lsblk -S says it's /dev/sda, use gdisk /dev/sda.

gdisk is a commonly used partitioning tool, similar to parted/GParted and to the Linux fdisk. The only difference is that it always writes out GPT partition tables – when it's run on a MBR-partitioned disk, it'll read the existing MBR partitions, but will always write the table in GPT format instead.

While in gdisk, don't do anything else (do not try to create or delete partitions), just directly use the w command to save the changes. (You don't need to create the EF00 partition; Windows Setup will do that anyway.)

Although if you want, you can use p to see the actual partition table and write down the start+length of each partition (in sectors) – if a partition is deleted but then re-created at exactly the same position, it'll still contain the same data as it did before (unless the partitioning tool deliberately cleans it out, which gdisk generally doesn't do).

After confirming the write, the old MBR partition table will be removed, and a GPT partition table containing the same partitions will be created.

(Yes, I realize this is mostly the same as in the linked post.)

because I don't have Windows

But you're already running Windows. The message

"Windows cannot be installed on this disk the selected disk has an MBR partition table, on EFI systems windows can only be installed on GPT disks"

is output from the Windows installer which runs on Windows PE. It isn't the full-fledged Windows but still the same Windows kernel with almost the same capability

That Windows PE can run normal Windows exe files and more importantly contains MBR2GPT.EXE for the lossless conversion. So after booting the installation disk just press Shift+F10 and the command prompt will open for you to do anything you want. Now just run mbr2gpt to do the conversion, for example the below will convert disk 0 to GPT

MBR2GPT.EXE /convert /disk:0 /allowfullos

As long as your disk satisfies the requirements, it'll convert the disk from MBR to GPT without data loss

See also Convert MBR to GPT - Without any OS

If it fails to convert then see Cannot convert from MBR to GPT for other solutions

For more information about Windows PE see Why is WinPE called an operating system?