Per the 10.3.1 Release Notes:
Expanded support GPT-formatted disks for disk encryption
Encryption is now supported on GPT-formatted removable and internal disks on systems with Microsoft Windows 8 and UEFI pre-boot authentication on systems with Windows 8 booting in UEFI mode. Encryption is also supported on GPT-formatted disks on systems with Microsoft Windows XP and later for removable disks.
This is a bit confusing, I agree. You should be able to use GPT on a secondary, internal drive with Windows 7 32-bit provided the boot disk is still BIOS/MBR.
Release notes cover everything we run through QA, so it is possible that this works fine, but was not sent to QA before release to test. As with any encryption, make sure the data is backed up before you encrypt the drive.