I've driven the Alaska highway a few times, but haven't biked it. Hopefully some riders will jump in, but until then...
The Alcan doesn't really have any terrible grades, and especially nothing like the Alps (I've biked Galibier, Alpe d'Huez, etc.). I think the maximum grade is something like 10%, and not for a very long distance. The grades are basically when you are coming in and out of river valleys. If you've cycled a lot in the Alps and can deal with that, you'll be fine on the Alcan. I'd characterize the Alcan as undulating with the occasional steep climb.
Check out the "Alaska Highway Bible" at www.themilepost.com
. It's a big, printed book, but is well worth getting for planning any trip in the far north, as it has literally mile-by-mile guides to most all of the northern roads. Really helpful.
There are also quite a few trip reports on crazyguyonabike.com. I'd start reading them to get an idea of the conditions.