For the grilled shrimp: Prepare a grill for medium heat. In a bowl, combine the lemon juice, garlic powder and a pinch each of sea salt and pepper. Add the shrimp and toss to coat well. Thread the shrimp onto metal skewers.
Lay the shrimp on the hot grill and cook until cooked through (the flesh will turn opaque) and grill marks form, about 2 minutes per side. Remove and set aside. When cool enough to handle, pull the shrimp off the skewers.
For the vinaigrette: Add the lemon juice, garlic and a pinch each of kosher salt and pepper to a large bowl. Whisk together, then slowly stream in the olive oil while whisking. Taste and adjust the seasoning as needed.
For the salad: Add the lettuce, avocado and tomato to the bowl with the dressing. Toss to coat well, then transfer to a serving bowl or platter, leaving any leftover vinaigrette in the bowl. Top the salad with the olives, goat cheese and shrimp. Drizzle the leftover vinaigrette in the bowl over the shrimp. Serve immediately.