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Along the lines with weight, your question about gearing and gradient really does come down to how much your bike weighs, it would be neat to have a graph or something to give you an idea of what an average rider (whatever that is???) would want for gearing for a 15% hill + a bike that weights "X" weight, but there isnt one.
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Just an aside ... Google Maps does a pretty good job at estimating the time for "an average rider" for a given route. Google doesn't know our weight, riding position or bike (but don't put it past them eventually), so it's necessarily a really broad estimate. Broad estimate is all OP is needing however.
Math works. Plug the hill part in to Maps, see how long Google says it will take, figure the speed from that, plug the speed, weight and grade into the calculator for a reality check on the power required.
Time, distance, grade and weight gives you everything you need for "what-ifs" using one of the online calculators. Just say "I'm going to put out 160 watts on the climb" and see what that speed is. Figure cadence from the speed and gear inches. If the cadence is way low, there's a problem.
Ps, I just did a spot check using 250 pounds & 15% grade, and 170 watts. You're climbing at 2.2 mph. With 19 gear inches that's a cadence of 40. That looks to me to be pretty difficult to sustain, very low speed very low cadence.