How can I get the uncompressed size of gzip file without actually decompressing it?

Please find my OS details:

$ uname -a
AIX xxyy 1 6 000145364C00

I've tried the following command to get size of a file in gzip archive:

$ gzip -l mycontent.DAT.Gz
compressed  uncompr.   ratio   uncompressed_name
-1223644243 1751372002 -75.3%  mycontent.DAT.Gz

Not sure how to interpret the unzipped size from this. Compressed file size close to 4 GB.

So, I tried this option in order to capture correct data:

$ zcat mycontent.DAT.Gz | wc -c

It gives me this error:

mycontent.DAT.Gz.Z:A file or directory in the path name does not exist.
0

Can you please tell me how to capture this value from shell script without decompressing the source file?

To answer the question title:

How can I get the uncompressed size of gzip file without actually decompressing it?

As you obviously know, the option -l (--list) is usually showing the uncompressed size.
What it shows is not calculated from the data, but was stored in the header as part of the compressed file.

In your case, the -l option does not work for some reason.
But it's not possible to 'measure' the uncompressed size from the raw compressed data - there is just no information about anything else in the compressed data - which is not surprising, as the point of compression is to leave out anything not needed.

You do not need to store the uncompressed data on the disk: zcat file.gz | wc -c is the right approach - but as @OleTange answered, your zcat seems to be not the one from gzip.
The alternative is using the gzip options -d (--decompress) and -c (--to-stdout), combined with wc option -c (--bytes):

gzip -dc file.gz | wc -c

Your zcat is not GNU zcat but from compress. Try:

gzcat mycontent.DAT.Gz | LC_ALL=C wc -c
gzip -dc mycontent.DAT.Gz | LC_ALL=C wc -c

I like using pv as it shows a more human readable information and progress:

zcat file.gz | pv > /dev/null

Outputs:

7,65GiB 0:00:44 [ 174MiB/s] [

Unfortunately, the only way to know is to extract it and count the bytes. gzip files do not properly report uncompressed data >4GB in size. See RFC1952, which defines the gzip file format:

ISIZE (Input SIZE)
    This contains the size of the original (uncompressed) input
    data modulo 2^32.

This discrepancy might be a little more obvious if whatever version of gzip you are using didn't have a bug: it is treating the ISIZE value as a signed 32-bit integer (resulting in -1223644243), rather than an unsigned 32-bit integer (which would result in 3071323053).

The most you can determine based on the header alone is that the real size of the uncompressed data is

(n * 4,294,967,296) + 3,071,323,053

where n is some whole number.