This is probably the most disappointing answer possible, but it depends on what you define as "easier" or "harder".
If you define difficulty based on the force it takes to keep the bike moving, it's certainly true that peddling uphill could be easier if you put the gears on a higher ratio. Frictional forces like air resistance and rolling resistance don't depend on the gear ratio, so using a higher ratio would help you combat these forces as well as the force of gravity.
If you define difficulty based on the energy it takes to get from A to B, the answer is less clear. If A and B are separated by a vertical distance of "h", the energy it takes to move an object of mass m from A to B is always going to be greater than mgh. There's no way around this, no matter which path you take or how cleverly you move the object. So in an ideal world with no friction, going uphill will take you E=mgh, and traveling level will take E=0. It's possible that when going uphill, frictional forces on the bike change so that the total resistance to the bike's movement is lower than on level ground. For instance, if you're going more slowly while traveling uphill, air resistance would be lower. This might mean it takes less energy to go uphill, but it could be equally true that frictional forces don't change enough to make up the mgh loss, meaning it would take more energy.
Another option is that you can define difficulty based on how easy it is, subjectively, to go from A to B. Again, there's no clear answer. At least for me, subjective difficulty isn't proportional to force--riding a bike for 10 minutes isn't as easy as riding it for 5 minutes, even if I'm applying the same force throughout the ride. It isn't proportional to energy, either--I find it much easier to bike up a hill by circling around it rather than going straight up, even though it actually takes more energy to circle around due to frictional losses. I suspect subjective difficulty also varies from person to person, so there's no easy answer to this question.
