In a bowl, whisk together tamari sauce, maple syrup or honey, apple cider vinegar, minced garlic, ground ginger, olive oil or sesame oil, salt, and pepper.
Place the tempeh slices in a shallow dish or container, and pour the marinade over them, ensuring they are well coated. You can also add some additional water if needed to cover the tempeh.
Cover the dish or container with plastic wrap or a lid, and let the tempeh marinate in the refrigerator for at least 30 minutes, or up to overnight for best flavor.
Line a baking sheet with parchment paper or lightly grease it with oil to prevent sticking.
Arrange the marinated tempeh slices on the baking sheet in a single layer, making sure they're not overlapping.
Bake in the preheated oven for 20-25 minutes, flipping halfway through the cooking time, until the tempeh is golden brown and slightly crispy on the outside.
Once baked, remove the tempeh from the oven and let it cool for a few minutes before serving.
Serve the gluten-free marinated tempeh with your favorite accompaniments or use it in various dishes. Enjoy your baked, gluten-free marinated tempeh!