usepackage 'optional' options

[Karl Berry]

    There is a ifplatform package which allow to test for windows and
    macos. It should work in your case. 

Yes, ifplatform is fine for Windows vs. Unix, i.e., the problem at hand
(although the gnuplottex package itself would ideally do the test). For
anything more specific, though, ifplatform needs --shell-escape, as I
understand it, which is IMHO highly undesirable.

So, for the sake of the curious and the archives, let me put in a plug
for Nicola Talbot's texosquery,
  https://ctan.org/pkg/texosquery
which provides an interface not just for os info, but also locale info,
directory listings, and plenty more, and it's safe (on the
--shell-restricted list), because Nicola put in a lot of effort to make
it so. --best, karl.