An armorer is someone trained to bring a specific type of gun back up to spec. by diagnosing problems and replacing parts that are worn or broken. A Glock armorer has been trained on Glocks, knows how to repair anything that can go wrong, etc. Ditto SIG, S&W, etc. Some armorers are trained in the upkeep of many different guns. They all know a lot more about guns than the average shooter, but they're not gunsmiths.
A gunsmith has these same basic skills, but has additional experience or training, can work with a full range of guns, can diagnose problems across the board, can fabricate parts, typically has metal-working equipment, may refinish guns, etc.
If an armorer is a "mechanic," then the gunsmith is also a mechanic and a bit of an engineer, as well.