It's funny when you notice that, while bikes are great on a level surface they're absolutely awful at anything moderately steep. Yes, even a multispeed bike.
Doing it on foot sounds easier. Unless you're talking about a very shallow inclination
A lightweight bike is ~6kg, what you gain from not having the impact of each step and being able to control your heart rate while keeping your cadence optimal via gears IMO more than compensates for the extra weight.
Standing takes a lot more energy than sitting - remember, the challenge is coming down again as well on each lap, but even on the uphill not having to support full bodyweight is an advantage, instead you just push with the effort you require on the pedals.
And of course if you are in the wrong gear a bicycle will be ineffective... that's why you choose the right gear?
Interesting that people use "shallow" as the opposite of "steep" (for an ascent), when it isn't really, and certainly it sounds odd to say "a shallow hill". I prefer "gentle", but I've always wondered if there are other languages that do have a specific single word meaning "not steep" that you can use to describe mountains etc.
At any rate, it depends a lot on the surface, the bike, the length and the sort of muscle development you have in your legs. For me on a decent road bike on a smoothly paved road, anything up to about 15% for a km or so is definitely easier than walking - I can do it a lot faster for a lot less effort. But on a loose gravel/heavily rutted surface, I'd probably have trouble staying upright for long at 15%+ on a typical road bike, and even 15% on a smooth road for much more than a kilometre gets very tiring (accepting it's pretty tiring to walk too, but it seems to put a different sort of strain on your muscles). I'm not sure if you'd consider 15% "very shallow" (I certainly wouldn't call it "very gentle"!).
Don’t forget about descending, which you will do a lot of in this challenge. Descending on your feet takes quite a bit of effort to control and stabilize your descent, and is also relatively high impact. Descending on a bike, on the other hand, requires basically zero effort aside from squeezing the brakes.
It's funny when you notice that, while bikes are great on a level surface they're absolutely awful at anything moderately steep. Yes, even a multispeed bike.
Doing it on foot sounds easier. Unless you're talking about a very shallow inclination