Gumball has not one, not two, but three fragments (*) of the original source assembly/code left on disk!
Here is my writeup about how to (re)discover them, extract them, and view them.
https://github.com/Michaelangel007/apple2_gumball
A _complete_ whole track of source code. Who knew!
(*) Fragments 1 and 3 are assembly, Fragment 2 is a symbol table.
Enjoy!
P.S.
Thanks to qkumba for the gentle reminder to get off my duff. =P