In a shallow dish, whisk together soy sauce, vinegar, corn syrup, hot sauce, and pepper. Add steak, bell peppers and onion and toss to coat. Cook immediately or cover with plastic wrap and refrigerate up to 24 hours.
Set a large nonstick skillet or stove-top grill pan over medium heat. Spray with cooking spray and add steak and vegetables, with the marinade. Saute 5 to 7 minutes, until vegetables are tender-crisp. Transfer mixture to a serving platter.
Meanwhile, warm tortillas in the microwave or according to package directions.
To make the guacamole, in a zip-top bag or medium bowl, mash avocado lightly. Then add tomato, garlic, lime juice and cilantro and combine. Season, to taste, with salt and black pepper.
Serve fajitas with guacamole on the side.