The Gable Creek Campground in the US is a backkcountry campground.....from Chief Mountain trailhead it is downhill 740 feet in first 1.8 miles and then flat along a creek, there are mountains around you that you can see....you follow the creek for 6 or 7 miles to the backcountry campground....it is right near Belly River Ranger Station....a nice walk but not much compared to most of the hikes in the Park....you can have fires there but there is nowhere to buy firewood, you must gather....you also must store food on poles or in bearproof lockers at backcountry sites....you also need a reservation to go backcountry camping.....
Gable Creek is a nice place to camp. The ranger station is not far away and the views up the valleys are very scenic. The only drawback is that the site itself has no major attraction like a lake or waterfall. And then there is that hill on the way out.
Gable Creek Campground is roughly a seven mile hike one way. Despite the "hill" on the return to your vehicle it is a relatively easy hike. If you just want to try backpacking with a seven year old for one night it is a good destination without too long of a hike. It is also fairly easy to reserve a camping site at Gable Creek - that site is usually not in great demand. However, Gable Creek does not compare to many other Glacier back country sites in terms of scenery.
You may want to start early and get your site at the campground and do some exploring in the afternoon. Make sure to check out the Ranger Station. If you head towards Cosley Lake you will cross a suspension bridge over the river. A bit further up the trail is Gros Ventre Falls - a nice place to spend some time.
Given the option, I would chose something like gunsight lake far before gable. Maybe even upper two med lake. The trip in is dull until you get close and the views open. Belly river station and sokme short hikes to the falls are good and mentioned above (dawn mist falls is a great side trip) Trip out will leave you with a final uphill push of approx. 600 ft.
I prefer gunsight as you have the lake, the views up the pass and a short hike to the base of a glacier. This is a 5 mile hike in, and 500 ft elevation gain. However, trip out leaves a 500 ft up at the very end.