In a bowl, combine soy sauce, brown sugar, sesame oil, garlic, ginger, rice vinegar, and gochujang. Mix well to create a marinade.
Add the sliced flank steak to the marinade, ensuring all pieces are well coated. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more flavor.
Heat vegetable oil in a large skillet over medium-high heat. Remove the steak from the marinade, allowing excess marinade to drip off.
Add the steak to the skillet in a single layer. Cook for 2-3 minutes on each side until browned and cooked through. Work in batches if necessary to avoid overcrowding the pan.
While the steak is cooking, warm the corn tortillas in a separate skillet or directly over a gas flame until pliable.
To assemble the tacos, place a portion of the cooked steak on each tortilla. Top with shredded cabbage, diced cucumbers, green onions, and cilantro.
Serve with lime wedges on the side for squeezing over the tacos.