[canucker2016]

But what source code debuggers did they have available?

Other than gdb, I can't name any Unix C source code debuggers. I believe they were working on Unix before GDB was created (wikipedia says gdb was created in 1986 - https://en.wikipedia.org/wiki/Gdb).

Plan 9 has acid, but from the man page and english manual, the debugger is closer to cli/tui than gui.

see https://9fans.github.io/plan9port/man/man1/acid.html and https://plan9.io/sys/doc/acid.html

[rascul]

Looks like there was an adb in 1979 which is supposedly the successor to db.

https://en.wikipedia.org/wiki/Advanced_Debugger