I found a solution to this problem. I went back and tried to install mysql-workbench using sbopkg. Much like before, it returned an error stating gtkmm was missing. I then tried to install gtkmm, using sbopkg. I got the error message saying atkmm was missing. So, I tried to install atkmm. This time I got an error message that stated
/usr/include/glib-2.0/glib/gtree.h:28:2 error: #error "Only <glib.h> can be included directly."
I did a search on the quote about glib.h. I found a handful of posts saying you had to go into the file and find every occurrence of an include with glib.h and remove the path to this file. So, if the path was #include <glib/glib.h>, it should be changed to #include <glib.h>. This itself didn't work. There is a conditional at the beginning of gtree.h checking for to variable constants. It returns this error, if they're empty.
This is a hack, for sure, but I commented out this conditional code and the atkmm install worked. And so did gtkmm and finally mysql-workbench.
I now have the MySQL GUI up and running. It seems to work fine.