Add the smashed, torn cucumbers to a colander and add the salt. Toss with your hands to distribute evenly. Weigh the cucumbers down with a large bag of ice. Refrigerate for 30 minutes.
Make the dressing. Add the gochujang, gochugaru, toasted sesame oil, rice vinegar, soy sauce, sugar or agave, and minced garlic to a small bowl. Whisk until well-combined.
Assemble the salad. Add the smashed cucumbers to a large bowl, and mix in the sliced shallots and scallion whites/light greens. Pour the dressing on top.