Thanks for all the great input!
I guess that's a pretty strong argument for Gwladus, but I have to say "Boo"!
I have a preference for Gwladys and Gryffydd. Further, I would use Marared, Llywelyn, Gwenllian, Elen (for Llywelyn ap Iorwerth's daughters), Dafydd.
I prefer the attempts to phonetically spell what was heard from the Welsh (of which I am ignorant). If those two names are pronounced in the way that I pronounce the anglicized names, Gladys and Griffith, "y" seems a better choice than "u". Actually, if Llywelyn is pronounced Lou-Ellen and not Li-Wellin then Llewelyn might be the spelling that does better justice to that name (sorry to bring that up Liz). I guess that would naturally lead to Elyn?
I am most familiar with FMG, but that source seems to have no problem with Anglicization! Short of documentation saying that Marared preferred to be called Margaret or actually wrote of herself using that name, I would not use Margaret. In addition to the scribes not knowing how to spell the names, the Anglicization of names was a matter of subjugation.
Are we aiming at a list of spellings we will use everywhere? Or will we be content to use Gruffudd, Gruffydd, etc., depending on the profile. I do not have a strong preference one way or the other. However, if we are going for a unified list, we better get about it!
Can we get RJ Horace or some other person knowledgeable about Welsh naming to weigh in?