I'm planning to hike the Long Range Traverse at Gros Morne National Park, which is a four- to five-day trek without trails. I have Arc'teryx Alpha SL pants, but I'm wondering if I should to bring gaiters, too.
I’m inclined to say that you'll want a pair of external gaiters. The Arc’Teryx Alpha SL pants ($198) are great, but I'm not sure you'll be comfortable wearing waterproof shell pants all day. Plus, they're made of Gore Paclite, which is a wonderful material but isn't the most durable stuff on the planet.
If I were you, I would carry the Alphas and wear some light insulated tights and shorts when it isn't raining. You might also look into Arc’Teryx’s excellent Gamma LT pants ($169), which are made of a light softshell that is rugged, comfortable, and water repellent.
For gaiters, Outdoor Research's Salamanders ($45) are a good choice. They’re a short gaiter made from a waterproof-breathable material, combined with Cordura in the lower section for excellent durability. When hiking off trail, the foliage is apt to be wet even if it’s not raining. That moisture can go right into your boot as you kick through shrubs.
If you expect a really wet trip, then look into a full-length gaiter, something like the Mountain Equipment Co-Op Kokanee Gaiter ($39, Canadian). They're made with Gore-Tex uppers and tough nylon lowers—just the thing for extended hiking in wet conditions.