We have experience on some of the roads that Google Maps chose. Specifically, we have used routes 98 and 160 between Kayenta and Page and Routes 89 and 12 between Kanab and Bryce Canyon. These roads are fine with some rolling hills - no mountain driving. We have never used the roads between Gallup and Kayenta but Google Maps in terrain view does not show them crossing mountainous areas. Road surface condition is another matter. You could use Google Street View to get an idea what the road condition and actual terrain looks like. Most folks seem to use route 89 between Page and Kanab, but we like route 89A for the scenery and for access to the Grand Canyon North Rim. 89A does have an ascent with 6-7% grade going up toward Jacob Lake and a descent further on going down to Kanab.
Bryce Canyon is beautiful. Make sure you get all the way out to the furthest overlook to see Byrce Amphitheater. Just WOW!