It’s easy to get drowned in technical details when it comes to gears but it all comes down to this: get gears low enough so you can pedal uphill at barely more than a walking pace.
These low gears are often called ‘granny gears’ and you’ll be happy you have them the first time a mountain looms in front of you.
Not only will you be able to climb calmly to the top, being able to spin quickly in a low gear takes pressure off your joints and prevents knee damage in the long run.
The range of your gears is measured in gear inches and a good range for a touring bike is usually about 20 inches at the low end to 90 inches at the top end. For more about gear inches, check out Sheldon Brown’s Gear Calculator.
Alternatively, look at the number of teeth on the chain rings. A standard set up is 3 rings with 22, 32 and 44 cogs on the front and back rings covering a range of 11-34 teeth. If in doubt, always give yourself a bit more range on the lower end than you think you’ll need. You’ll never regret having the ability to spin up that killer climb.
You may not want to think about gears at all, in which case you can invest in a Rohloff Hub; an internally sealed gearing system that requires almost no maintenance but is very expensive.