Interestingly enough, there is conflicting information on Kia Tech Info.

If you look up the repair procedures under the Engine Electrical System/Ignition System/Spark Plug for the 2001 model year (and maybe others, too), at one point in the "installation" section it states that the gap should be 0.039-0.043. It also states this in the specifications.

But further down in the inspection section, it states the gap should be 0.028-0.031.

I saw a moderator on another Kia forum recommend .030, so I went with that for now. If I open it back up again at some point, maybe I'll gap it back up to .042 and see if it still runs smooth.