It bothers me that I can't find a treatment of Gaelic grammar in Gaelic itself. So I've put together at least a little list of grammar-terms in Gaelic. At least half of this was dictionary-work, and I'm not sure on the details and nuances, but at least it's a start. So, if you're looking to talk about grammar in Gaelic, here's a starting point.
Gaelic Grammar Terms
There does appear to be a Gaelic Grammar: Facal air an Fhacal by Michel Byrne, but it doesn't seem to be available anywhere any longer.
There is also a list here of Grammar terms, which I had forgotten about.
There *is* a long out of print Gaelic language Grammar, with parallel English. It's The principles of Gaelic Grammar, by John Forbes, 1848. Some of the terminology and spelling is outdated, but short of a re-release of Facal air an Fhacal it's probably the best you'll get.