If you have not already done so, please read the handbook section about
graid(8).
https://www.freebsd.org/doc/handbook/geom-graid.html
This is different to the page linked earlier, which was for
gstripe(8).
And yeah, GPT is incompatible with many of the other
geom(4) classes for entire-disk mode. It is a bug that
gpart(8) silently lets you create a GPT scheme inside those classes (if it still does that, I've not tried it in a long time). They can be used with care inside individual GPT partitions, with the exception of
graid(8), which must be an entire-disk setup for the BIOS to properly deal with it.
MBR or BSD partitioning is the safe option in general, unless you need something from GPT (such as EFI booting, or 2TB+ drives). BSD64 partitioning is another valid but uncommon option for 2TB+ drives. As far as the mentions of "dangerously dedicated" for BSD partitioning go, it's only dangerous if you let other operating systems (or any non-BSD tools, e.g. from bootable diags or the BIOS) meddle with the drives, it's not dangerous on a system which only ever runs BSD. Many people happily use BSD partitioning on dedicated BSD systems without any issues.
The 8.0 release notes were very badly worded when they suggested that "dangerously dedicated" would not work, and did not give a true impression of what does and does not work; in case you find any references to "not supported" in that context. It was just removed from the installer to reduce the chance of novices shooting themselves in the foot. The actual ability of the OS to safely use dedicated BSD label drives was unchanged.