Hi @bakern, thank you for kind words about Mender and welcome to the community.
So the the directory structure that is created is compatible with UEFI but as you have noticed the output image is not an GPT image and this has been the case so far.
As you have noticed the TODO, this is something that we need to add.
The changes needed to create an GPT image are probably in these lines, https://github.com/mendersoftware/mender-convert/blob/master/mender-convert-package#L226-L233 and should not be much.