How come both "cath" and "gath" are recommended but only the former is accepted?
Gath is a mutated of Cath and therefore you won't find it in a dictionary.
Thanks for the answer. Does this mean it comes after certain phonemes or in certain syntactic situations? i.e. Should it never be used in isolation?
Yes you will only see gath in certain situations. The most common is probably after "Y/'r" which cause soft mutations in feminine nouns. I.E "The Cat" is "Y Gath".
Beware - the hints that pop up when you hover over a word are not "recommendations".
Also, they are not sentence-specific: the hints for a given word will always be the same list, regardless of which sentence that words pops up in. Many of the hints will only apply to some sentences but not to others. (For example, the hints for the English word "lie" might be something like "tell an untruth" or "be horizontal", but in sentences such as "Don't lie to me" or "We lie on your couch", one or the other would be inappropriate.)
Similarly here: in some sentences, "cat" is rendered by gath because the words around it cause lenition (soft mutation), but in others (including this one), it is the basic form cath that is appropriate.