Thanks for this reply.
"some fonts do not work" : Is this caused by a bug in Lr?
Is there an explanation for this?
I don't know. I can not see why these fonts do not work. Perhapst some (internal) information is missing, which LR is expecting.
You could create a PNG of your watermark using the Gabriola font in Photoshop, then use the "Image Option" instead of the "text option" to insert your watermark.