If it's your first visit and you live in a less mountainous country, it can be hard to know what gearing you need for a road cycling holiday in the Pyrenees (separate post for gravel riding to follow). When your rides tend to feature short climbs of 5-10 minutes, you can afford to let your heart rate climb because you'll soon be over the top and able to recover. In the mountains, you have to settle into a rhythm that you can sustain for well over an hour.
The simple answer is to go low - it's way better to have gearing that’s a little too low than too high. And if you're less fit, or perhaps not quite as skinny as the pros, then of course you need to go lower still.
A good way to test is to find the longest hill near to where you live, 10 minutes and around 8% ideally, and to ride repeats of it until you have totalled 30 minutes of climbing. Keep the pace steady, as if you're on a big day in the mountains. The aim is to have a gear that allows you to maintain a manageable effort and a comfortable cadence even on the last ascent.
I'd say that, even if you’re really fit, 34x28 is the minimum. And if you're less confident in your fitness, then 34x32 is a good bet. There are some really steep sections on many of the climbs and you will appreciate being able to tackle them without resorting to a lung-searing effort or knee-bursting low cadence. This is no place for a 53/39 chainset!
If your rear derailleur has the capacity, a cassette that goes up to 32t or 34t will make a significant difference and is the most affordable way to lower your gearing. Shimano now offers an 11-34 cassette at all levels, creating a 1:1 gear, and 11-36 in 105. Remember that if you fit a larger cassette you will likely need to extend or replace your chain, or be very diligent about avoiding cross-chaining into big-big.
Campagnolo now offers a 1:1 bottom gear in its Super Record and Record groupsets (50/34, 11-34), and an even lower 48/32, 11-34 option in Chorus, and all are already 12-speed so the gaps between gears are reduced.
SRAM continues to lead the way with ever lower gearing. The Red and Force eTap AXS offers a sub-compact 46/33 chainset and 10-36 cassette, giving a lower than 1:1 gear, and the Rival eTap AXS groupset goes down to a winch-like 43/30, 10-36. The lowest of those options are intended more for gravel use, and you would certainly be glad of them out here.
On the road, a 1:1 gear or lower makes almost any climb possible for almost any rider, but you have to remember that you will be travelling more slowly and a major climb could take well over two hours. The likes of the Col du Tourmalet, 19km on the Luz side, could represent an endeavour beyond three hours. Something to keep in mind when planning your days.
A 1x drivetrain - that is, with a single chainring - will be at its most limiting in the mountains where you will experience a greater breadth of speeds than anywhere else. Yes, you can achieve the range with, say, a 44x10-42, but the jumps between gears will be unpleasantly large. Alternatively, a less gappy cassette will limit the range and you'll either be grinding uphill or unable to pedal downhill even on the gentle valley descents. It can work, but it's compromised.
