This is one of those "ask 10 different people, get 10 different answers" kind of questions, but I always build my wheels with the same guage spokes on both sides, and they are usually DT Competition 14/15/14. Some people like to run thinner spokes on the non-drive side to save weight, and others have some kind of practice based on the drive side spokes requiring more tension. I just like to keep it even for simplicity, plus it hasn't been a problem yet.
Some people have pointed out that using different spokes on the drive side of a dished wheel (a different gauge from those on the non-drive side) makes for a more durable wheel.
The application I am most interested in is long distance touring, with heavy loads.
DT Alpine III spokes have been r e c o m m e n d e d. Would using two different gauges make for a better wheel?
The logic of using different gauge spokes to make a more durable wheel escapes me. Marginally lighter wheel, yes. More durable wheel, no.
As far as marginally lighter is concerned, assuming that you could save 2 grams per spoke on a 36 spoke wheel by using smaller gauge spokes on the non-drive side, you would save a total of 36 grams (18 non-drive side spokes x 2), for a grand total weight savings of just over 1 ounce.
Inasmuch as your intended application is for heavily-loaded touring, I would stick with the conventional same-gauge spoke method.
I build my own wheels and use same-gauge spokes on my rear wheels.
Highly dished rear wheels begin with very light tension on the left side spokes. Those lightly tensioned spokes fail more frequently because as the wheel is ridden, they can cycle in and out of tension, repeatedly going completely slack. One workaround is to build the left side of the wheel radially. This increases the required tension on the left side somewhat and eliminates the presence of leading spokes that are more prone to slackening under drive torque.
Wheels built radially are more prone to hub flange failures. There is less flange material to support spokes when they are pulling straight out instead of tangentially. Furthermore, old hubs that are re-built radially are even more likely to fail due to stress concentrations in the notches left from the old spokes.
Another way to aid highly dished rear wheels is to use thinner spokes on the left (non-drive) side. In such a wheel, the lighter left-side spokes are under the same tension that regular left-side spokes would carry. The thinner left side spokes are elastically stretched further than thick spokes would have been. During cyclic loading, the thinner spokes have farther to "un-stretch" before they go slack.
Anecdotally, you may have heard wheelbuilders say that the key to a reliable wheel is tight spokes. What's really meant is that the key to preventing spoke fatigue is to eliminate cyclic un-tensioning of spokes. Super tight spokes are one way to achieve this. Another way is to spec "stretchier" spokes where the spokes are lightly loaded and/or more likely to cycle completely out of tension.