Why was the dependency injection pattern not incluided in the gang of four? Did GOF pre-date widespread automated testing? Is dependency injection now considered a core pattern?
I was Editor of Software Development magazine when the Gang of Four book came out and I can say with total confidence that unit-testing was not a widespread practice in 1994, when Design Patterns was originally published. 

In 1994, C++ was the most commonly used object-oriented language, and most people programming it were coming from a C background. One of the "thinking in objects" things that people simply didn't have is the idea of hundreds or thousands of entry points into your program. You thought about the main(). If you worked on a large project, you might have a (usually quite elaborate) makefile to create a module-based program. But "unit-testing"? Starting a process, building the necessary memory context, executing it, and tearing it down, on a per method basis? That was very radical.

Java made multiple-entry-point programming more obvious. By the time of the original Dot-Com boom, unit-testing was a well-known technique, but it was really JUnit (circa 2001?) that caused it to catch fire and become a universal practice.

Although Strategy and the general concept of programming to an interface were part of GoF and the mid-90s zeitgeist, the idea of injection came quite late to the party (circa '03-'05?). Honestly, my gray hairs are still quite dubious about that aspect of DI ("Get off my lawn, you darn configuration files!"). 
