Basically I think Gnuplot does not need special rights to install or execute, so it should be possible to install it in some folder and use it.
Currently you cannot install (Gnuplot 6.0) on Windows without administrator rights.
Other programs like Gpg4win (https://www.gpg4win.de/index.html) allow non-admin installations (and use), too.
At the moment, please use zip or 7zip packages for non-admin instalation.