Although, I have started switching from Adobe a few years back, in your case Adobe guy might be right to ask you to upgrade to High Sierra. Each new version of Mac OS X (macOS) automatically instals new/updated printer divers. When you don't have the right driver installed, macOS allows you to print through GIMP but can not offer all the options available through that specific printer's driver.
It might also help to know that even a periodic Sierra update should have provided/updated your printer's driver but it is common of manufacturers — in your case Epson — to not care to provide new drivers or their updates to Apple until a new macOS version is due out. Hence, all the hassle.
Need to leave or subscribe to the Sciart-L listserv? Follow the instructions at