I have been working on an "optimal" solution over my three Caminos: 2013, 2014, 2015. During these three pilgrimages I have tried trekking raincoats with rucksack "humps," gaiters, rain parkas combined with pack covers, and ponchos. The only combination I have not, nor will I even consider using, is a full pants and parka rain suit. Here is my considered opinion on the merits of each approach:
Trekking raincoats:
They work marvelously, especially in the "hip" seasons (April - May) and (October - November) they provide needed warmth and near total protection from the rain or snow. They are also highly effective at blocking wind. However, they DO cause excess perspiration that condenses INSIDE the raincoat. This means that regardless of the built in ventilation, when you stop and take it off, both you and your rucksack WILL be wet or soaked through with sweat instead of rain or snow. My recommendation if you like this style is to use it in conjunction with a pack cover or pack liner to keep your stuff dry (I prefer the cover).
Gaiters:
Sorry, but I am not convinced this is value for money spent or weigh carried. I tried these on my first Camino. MY finding is that wearing synthetic, fast drying, zip-off trousers, was a superior solution with no weight penalty. When sodden from rain or snow, the trousers dried within 30 minutes while walking. Once coated in mud, I just unzipped the lower portion and hand-washed them, rinsing the excess mud off in a shower. This left me able to wear the top portion as shorts. Simply wash, rinse, and ring them out very well, twisting one way then the other, hang to dry, and they should be at least damp but usable in less than one hour. I personally recommend not spending the time or money gaiters command. They make the most sense if walking through knee-deep snow during the winter. Rather few pilgrims walk the Camino during these conditions.
Rain Parkas with Pack Covers:
This combination is fine for light, sporadic showers in later May or October. However, in a heavy rain, rain will work between the outside back of the parka and into your pack through the uncovered back panel and around the elastic edges of the pack cover. Water does have a unique property to wick and find it's own way in. If you are going this route I recommend two things: (1) Get a parka with arm-pit zippers for optimum ventilation, and (2) use a bin bag or similar waterproof liner for your pack. The rain WILL get in and soak the pack material.
Ponchos:
I have come to the personal conclusion that a "modified" hiking poncho, one with a "hump" for a rucksack is the way to go, at least for me.
First, I wholeheartedly recommend siliconized nylon (or sil-nylon) as a very good, lightweight material.
Second, I have, and recommend "sealing" the snaps so they do not easily come free in wind or while moving. I first used duck tape, but that tore with hard movement. Then I used 3M reflective tape, cut into 1" (3 cm) wide strips to place over the snap like a "U." This worked, however the snaps could still become "unsnapped." The tape held the poncho closed.
My next iteration is going to be to super glue the snaps closed, then apply reflective "dots" I obtained in a Decathlon in Belgium on all 16 snap surfaces. There are 4 snaps on each side of my Sea-to-Summit Sil-Nylon poncho. Each snap has 2 sides. So that is 8 dots to each side.
Third, The reason I prefer a poncho, with a pack cover and a ball cap, is that it provides maximum rain protection together with maximum ventilation. The entire issue of securing the snaps closed is because, walking a Camino, a pilgrim usually never needs to use the poncho for anything other than rain protection. Yes, hypothetically, one might need or desire to open the poncho to use as a ground-cloth under a tent, expedient awning, or even a "blanket" in an albergue. However, in my experience, these are extremely rare occurrences. I simply assessed my needs and Camino-style and adapted my gear choices accordingly.
Additional Important Point That Runs Across Comments:
Regardless of the rain gear solution you choose, do wear a ball cap or visor, as discussed above, to improve forward and lateral vision while walking. Cinched-down hoods, on ANY rain gear causes tunnel vision, I almost fell off Alto de Perdon at the steep drop of section just before the sculptures, west of Pamplona, in 2013 because of this. They have since installed a barrier cable to prevent the sort of accident I nearly had. I use a brightly colored, synthetic material, mesh ventilated cap from Outdoors Research (
http://www.outdoorresearch.com/en/accessories/hats/swift-cap-4.html). However, I have seen comparable caps for a fraction of the cost at Decathlon.
I sincerely hope this helps.