I always preferred this ever so slightly to Weirdstone, and one of the reasons may be that Colin and Susan have a little more agency in this book, while at the same time having less. More stuff happens to them directly and they do things and even have opinions, but they remain, sadly, ciphers, albeit ciphers on the cusp of change. More than that, though, it was the idea of wild magic, magic that exists purely for its own sake, savage and emotional and dangerous, set against the more ordered, courtly magic of Cadellin, which anticipates a lot of modern fantasy magic with rules and systems, but of course, it is the wild magic that breaks Susan's heart at the end, and leaves the reader haunted too.
Gomrath is a wilder, more formless book as opposed to the rather tidy chase narrative of Weirdstone. The magic comes out of the very landscape, and the danger from the shadowy Brollachain and the shape-changing Morrigan while Colin and Susan's relationship with their allies is more uneasy, and strained to the point of bitterness with the lios-alfar. Futhermore, much is left unsettled at the end, unless I missed some details, with the Morrigan still on the loose and whatever was bothering the lios-alfar unresolved. In retrospect, the set-up for a third volume was always there, but Garner resisted or refused, and many years later we got Boneland, something of an entirely different order.