Just spent way too long thinking it was something wrong with my SSH configuration. Ugh. The former X11.app is now an open source project that needs to be independently installed. Once installed things Just Worked the way they used to. An example error message that is resolved, that I encountered when trying to run
Application initialization failed: no display name and no $DISPLAY environment variable
Error in startup script: no display name and no $DISPLAY environment variable
"load /usr/lib/libtk8.5.so.0 Tk"
("package ifneeded Tk 8.5.11" script)
invoked from within
"package require Tk"
(file "/usr/bin/gitk" line 10)