Been doing some research.
There is no GCC Elite XL 20/600 support in GutenPrint, Foomatic, HPIJS, or even Ghostscript 9.21. I also cross-checked with any of these aftermarket solutions supporting the Apple LaserWriter 8500 as I believe that was an OEM GCC Elite XL. Nothing there either. I have the LaserWriter 8500 v1.2 PPD, but not the corresponding LaserWriter 8 driver that uses it.
If you also have a spare Windows XP, 7, and possibly 10 networked machine, you might be able to install GCC support on that machine, and share that printer with Sierra via these Apple instructions.
Also, I wonder if when you are at the Sierra Print panel, if under Paper Size, do you have a selectable Manage Custom Sizes... menu entry, where you can create a custom Tabloid paper size that will then become available to you in subsequent print panels?