These days I keep on seeing repeated shout-outs to gochujang, which is not available where I live. However, I do have a container of Doubanjang just sitting there in my fridge, taking up space. Would it be a reasonable substitute in recipes calling for gochujang?
Both are mildly spicy, but the similarities end there. Gochujang is tangy and slightly sweet, whereas doubanjiang is more salty, savory and fermented-tasting. Even the textures don't match up: gochujang is smooth, while doubanjiang is chunky and ragged. I wouldn't substitute either one for the other.
Incidentally, if your doubanjiang is just sitting there because you got it for some elaborate Sichuan recipe involving like eight other seasonings... doubanjiang is more flexible than that, and will work well as an addition or even just on its own. Try frying up some ground pork with a bit of doubanjiang and some scallions, served over rice.
As @Sneftel has said, the flavor profile of doubanjang is too distinctive to serve as a replacement for gochujang in any real context.
That said, the flavor of gochujang is relatively easy to compartmentalize, and thus reasonably replaceable. The uses of gochujang in Korean crusine stems pretty much directly from its constituent ingredients:
|Gochugaru||color and hotness||can be replaced by gochugaru, or any type of red pepper flakes with a neutral aroma profile|
|Malt or sugar||sweetness||can be replaced by sugar, honey, or other sweeteners|
|Rice flour or other starch||thickening||a bit of potato starch or flour (typically not very important)|
The meju and additional fermentation does impart some savoriness and fermented flavor, but Korean recipes will typically call for doenjang or even ssamjang (which is in essence doenjang and gochujang combined) if those flavors are significantly desired.
So, depending on the dish or even personal preference, a combination of ingredients that can impart the all or some of the above aspects will generally work quite well (although red pepper flakes are de facto mandatory) - for example, people who prefer a cleaner "broth" in their tteokbokki may use only gochugaru and sugar in place of gochujang.