Hi Rick,
"ISIZE (Input SIZE)" from https://tools.ietf.org/html/rfc1952 looks promising for most GZIP archives containing a single file.
Yes, this field should be the one that is discussed in the StackOverflow entry. – As the field is limited to values of 2^32 bytes, the file size won’t be correct for files >4 GiB, so a more generic solution might be to count bytes while parsing them, and sum up the processed bytes after parsing.
N.B.: The Database Resource Properties INPUTSIZE for ZIP archives also shows "0 b".
I was surprised to read this. Once again, it’s due the contents of the NIST ZIP archives that don’t contain file lengths (just try some other ZIP archives to see the difference).
Best, Christian