From VT (and from the same text that mentions Aʒūlēz).
'The former presence of initial g could be detected by the comparison of, say, Q. alda 'tree' with T. galla, and the process of loss be deduced from the spelling in the old Rúmilian script ʒalda (using an initial sign which was known by tradition among the loremasters to have represented the open back spirant).' JRRT
That's from
Quendi And Eldar Appendix D published in Vinyar Tengwar 39. In
Return of the King it is noted:
'gh in the Black Speech and Orcish represents a 'back spirant' (related to g as dh to d); as in ghâsh and agh.'
Just to source a couple things (outside of the web).