How portable is a gzip file over 4 GB in size?
I have just noticed that gzip has a 4 GB size limit
More accurately, the gzip format can’t correctly store uncompressed file sizes over 4GiB. The result is that gzip -l won’t show the right size for any compressed file whose original size is over 4GiB.
Apart from that, there is no limit due to gzip itself, and gzipped files over 4GiB are portable. The format is specified by RFC 1952 and support for it is widely available. (Strictly speaking, the gzip format stores the size of the uncompressed data modulo 232; it’s gzip -l that’s misleading, for this and other reasons.)
Will I be able to untar/gunzip large files anywhere?
Anywhere that can handle large files, and where spec-compliant implementations of tar and gunzip are available.
In other words, how portable is a gzip file which is more than 4 GB in size?
The gzip format itself is portable, and gzip files are also portable, regardless of the size of the data they contain.
Does it matter if I create it on Mac OS, Linux, or something else?
No, a gzip file created on any platform can be uncompressed on any other platform with the required capabilities (in particular, the ability to store large files, in the context of this question).
See also Compression Utility Max Files Size Limit | Unix/Linux.