A very good book on the subject is, "Northumberland Place Names" by Godfrey Watson (1995 Sandhill Press, Warkworth.) I cant find Gled Law in it, but think you are right, however, Mr watson thinks Cullercoates is derived from Culfre-Cotes, which are Dove cotes.
Other Celtic names he mentions, from Northumberland are: Yeavering-from Gefrin-hill of goats. (I have seen a herd of them up there)
Ross Castle Hill Fort--Ross is a promontary. and The Pennywell at Hulne Park, Alnwick derives its name from Pen-Y-gwel, the rampart on the hill.
He also sugests that many place names that cant be explained may well contain Celtic elements that are now impossible to trace.
It would be interesting to go back in time and find out what the pre-Anglo-Saxon names for other sites were--but I suppose we will never know.
Any more thoughts on the Duergar? I cant find any more info. and Rhiannon`s Tuatha theory does seem quite convincing....