GPT HDD on T3500

This is odd.

Is drive damaged?  What do you mean you "lost" 750G?

(also, why on earth are you installing your system on an enormous storage drive?)

I lost 750 GBs when I converted it to MBR to install windows on it, then I realized if I want to get the full capacity I have to convert it to GPT but unfortunately I can't install windows on a GPT drive since my PC doesn't support UEFI

Also, I'm using only 400 GBs for the windows because my SSD is damaged and I have a lot of work to do until I buy a new one

To convert a 2TB+hard drive from GPT to MBR, you might find some disk space is lost after conversion. That's because the MBR disk can only support a 2TB hard drive at most. In this case, you can open Disk Management (a windows native hard disk manager), where you will find the lost space is shown as unallocated space at the end of the disk. You can add it to the partition that's at the right adjacent side to extend the partition.

The boot drive in a non-UEFI, Legacy BIOS system like the T3500 must be initialized to MBR and not GPT.  MBR has a 2TB partition limit.  What you can do with a 3TB physical drive, though, is have a primary 2TB partition and a 1TB secondary data partition.  In Windows you'd see a 2 TB C: drive and 1 TB D drive.