Hello,
Yes Dan is correct on this. I can expand on the subject a bit since I have been in a sheet metal related business most of my life.......
First of all remember that sheet metal gauge is a rough indication of thickness. 18 ga. sheetmetal averages around "047" thick, but it can vary around .004" up or down in thickness and still be 18 ga. I am not saying that your sheetmetal will vary this amount on a single part. When the steel mills produce the metal the entire batches will be produced at a thickness and will fall into the specs for that gauge. Also realize that the gauge number goes higher as the metal thickness becomes lower. So a 22 gauge sheet metal would alverage around .029" thick. The Sheet metal in the body panels and non critical areas of the floor pans on these cars is around 19 ga. as indicated by Dan which is usually around .0418" . Also as stated much thicker sheet metal is often used in structurally critical areas of the chassis.
Also realize that the alloy and the temper of the steel can vary widley, depending on the manufacturing specs, this will also cause the strength and character of the steel to vary greatly. With this in mind it is always better to go slightly higher in thickness than thinner, when doing repairs or fabricating panels. So as Dan has related, repairs with 18 ga steel is slightly thicker than the original 19 ga metal.
One great advantage of the K&K repair panels is, unlike originals, they are zinc coated (galveneal), which greatly improve resistance to rust in the future. The zinc coating does make welding a bit more difficult, but it is well worth the trade off.
The Aluminum body panels on the cars is around "063". At edges (trunk , hood etc) the aluminum iis two layers thick, as inner and outer panels come together to form an nice ridgid edge nearly .125" thick.