In a shallow bowl or casserole dish, combine the balsamic vinegar, coconut aminos sauce, basil, minced garlic, and salt and pepper to taste.
Place the mushroom caps in the mixture and toss to coat well. Let soak for 15 minutes, turning every so often.
Spray or grease an indoor or outdoor grill.
Cook the mushroom caps for 5-8 minutes, alternating sides, until tender.
Add the lettuce and tomato to the bottom of a bun. Place the grilled mushroom on top, followed by the vegan gouda cheese. Place a ring of onion on top of that.
Spread some of the guacamole on the top part of the bun and place it on top of the burger to serve.