Sorry, I missed this earlier note from @grannyknot. Wish I could remember where I got my gauge chart from (it dates back several years). The discrepancy between GK's values and those from my chart are large, so I just went looking for another source and came up with this one from machinemfg.com (website for a machine manufacturer). I wouldn't call it a definitive source, but the values coincide with the chart that I posted...
www.machinemfg.com/sheet-metal-gauge-sizes-chart-inch-mm/#Sheet_metal_GAUGE_thickness_chart
So then I searched for a chart for the American Wire Gauge (AWG) and discovered that its values agree with GK's...
www.powerstream.com/Wire_Size.htm
None of my sources are definitive. If someone wants to post the tables from an accepted engineering reference like Shigley's, that would resolve the issue once and for all (I never could afford that book when I was in school and just used my employer's copy after I got out into the workforce).
It appears, though, that we've tripped over yet another facet of wacky world of 'gauge' measurements -- that is, that gauges have different 'thickness' values, depending on whether you're looking a wire diameter vs. sheet thickness. Another vote in favor of metric.