Epilogue: my wife loved the electric bike too. I put our kid's seat on the Gazelle and it made zipping my son around the neighborhood so easy and pleasant that we decided to get another so we could both ride them together. We're selling our old bikes (a road bike and a mountain bike) and going fully ebike aside from our kids' bikes. We weren't sure whether to get another full-sized one or the Gocycle like I'd initially intended, but in the end the flexibility of having one of each won the day -- being able to take either the small folding or the full sized is convenient enough.
So I was finally able to see what the range of the Gocycle GX is like
Picked it up yesterday afternoon and rode it around 11 miles. On the flats, I kept it in Eco mode, but on hills I cranked the assist up to 100% -- including the one under the GWB, which the GX does a very good job on -- I still need to pedal non-trivially and on really hot days I'll crack a sweat, but it's still an order of magnitude better than without assistance. The Gazelle in its lowest gear is noticeably more powerful. But after 11 miles, including 100% assist on the GWB hill, the battery was only depleted 20% (in other words, it had 80% remaining). Much, much better than I had anticipated!