The company I work for is a pay-for-subscription news website.
Is it legal to use QGIS to host maps that would be only available on our site to paying subscribers?
I'm having trouble figuring out the GNU license.
You can use it for everything you want - no restrictions apply.
You also can sell QGIS itself (you just need to find someone who's willing to pay).
But if you do (even when you distribute without charge), you have to include the source code of QGIS and all own extensions (plugins, features, whatever) you added. Basically pass on everything you got (and not restricting it by anything you add).
For more general GPL questions checkout http://www.gnu.org/licenses/gpl-faq.html.