Is There A Multiplier To Get From Sheet Metal Gauge To Actual Thickness Or Weight?

The data on this site appears to be piece-wise linear, having eight linear segments between 7/0 and 38 ga. Each higher numbered gauge is approximately 90% of the weight per square of the one before, but that approximation is not sufficiently close that it can be relied upon.

A spreadsheet should be able to compute a piecewise linear function using a lookup table.
7/0 ga  320 oz/ft² decreasing by 20 oz/ft² for each ga.
1 ga  180 oz/ft² decreasing by 10 oz/ft² for each ga.
14 ga  50 oz/ft² decreasing by 5 oz/ft² for each ga.
16 ga  40 oz/ft² decreasing by 4 oz/ft² for each ga.
20 ga  24 oz/ft² decreasing by 2 oz/ft² for each ga.
26 ga  12 oz/ft² decreasing by 1 oz/ft² for each ga.
31 ga  7 oz/ft² decreasing by 1/2 oz/ft² for each ga.
36 ga  4 1/2 oz/ft² decreasing by 1/4 oz/ft² for each ga.
38 ga  4 oz/ft²

You need to figure out the density to convert to thickness. It looks like 20 oz/ft² corresponds to about 1/32 inch thickness.