M. Tarenskeen is correct. When you install Ghostscript in Termux, you automatically get a whole slew of valuable utility programs with it, including ps2pdf. Once gs is installed, just type `which pf2pdf' and there it is!
Gnuplot supports both PostScript and PDF "terminals." Just select the one you like best. If it's PS, then ps2pdf makes short work of the PDF output. Choosing the PDF terminal bypasses the explicit conversion step. What I've done is put a symlink called "storage0" from my Termux home directory to /storage/emulated/0. I can copy the finished plot into the symlinked directory, and voila! It's right there, immediately accessible to any Android utility.
I happen to be running the FBreader ebook reader with a PDF plug-in, which so far is doing a nice job of displaying the finished PDF plots.
I currently am running my Samsung Galaxy J3 phone without an SD card, but as soon as I get one I'll try the same trick on "storage1".