If my motherboard does not support UEFI BIOS, can I still use GPT for non boot drives?

Non-boot GPT disks are supported on BIOS-only systems

It is not necessary to boot from UEFI in order to utilize disks partitioned with the GPT partition scheme. Therefore you can take advantage of all of the features offered by GPT disks even though your motherboard only supports BIOS mode.

With Windows, as you've already observed, the only restriction of concern is that you must boot in UEFI mode if you want to boot from a disk partitioned as GPT.

Although Twisty’s answer somewhat hints at it, I’ll spell it out: BIOS does it all.

In fact, GPT or MBR don’t matter at all. The BIOS will simply load the first sector of the HDD/SDD into RAM and start executing whatever is there. It does not know about partitions or MBR or whatever. GRUB supports this too, although it requires an additional partition (EFI type “BIOS Boot”) to fit all of its early boot code.

That means pure BIOS systems support GPT for both booting and any other use.

There is one tiny exception though: Microsoft has made it an (artificial) requirement that booting Windows from GPT requires UEFI.

Twisty and Daniel B have both provided good answers; however, there is a major caveat: GPT support depends on the OS in use. The question specifies "Windows Server" as the OS, but not the version of Windows Server in use. To be sure, all recent versions of Windows support GPT for data disks; however, older versions do not. The Wikipedia page on GPT provides details for various Windows versions. From those tables, only the 32-bit versions of Windows Server 2003 and Windows XP lack this support; however, I believe there may be some subtleties that these tables omit. (IIRC, Windows Vista picked up GPT support with its Service Pack 1, for instance. I might be mis-remembering that, though, so take it with a grain of salt.)

There is another caveat: Disk drivers sometimes impose their own 32-bit limits, which can make over-2TiB disks look like they're smaller than 2TiB in size. (Typically, you get a disk that looks to be a modulo of 2TiB, so a 3TB disk will appear to be under 1TB in size, for instance.) Problems like this used to be fairly common, but I've seen fewer posts about them recently. Presumably the bugs have been mostly fixed by now; however, you might still run into the problem, particularly if you're using an older driver stack. I recommend checking the disk's apparent size before beginning to use it. If the size doesn't look right, update your drivers.

Overall, then, you're probably fine. If you're using Windows Server 2003, you should have upgraded long ago for reasons other than GPT support. If you haven't, you no longer have an excuse to put it off any more.