Add the oil, garlic powder, lemon juice, salt, pepper and garlic powder to a gallon-sized freezer bag and mix well.
Add the chicken breast and seal the bag.
Toss to coat and then chill overnight or for at least 2 hours.
Cook the Chicken
Grill the chicken for 6 to 7 minutes on each side over a medium-high heat, or until cooked through with clear-running juices.
Let it sit for 5 minutes, then cut into bite-sized pieces.
Cook the Pasta
Meanwhile, cook the pasta following the directions on the package.
Drain and rinse under cold running water, then let it cool completely.
Assemble the Salad
In a large mixing bowl, toss the pasta, chicken, tomatoes, red onion, cheese and cucumber together.
Pour the dressing over and toss to mix.
Chill for at least 15 minutes before serving, for the best flavor.
Notes
Salt the pasta water: Cold pasta tastes a bit flat when it isn’t seasoned during cooking.
Drain pasta well: Extra water can thin the Italian dressing.
Toss before serving: Dressing settles at the base of the bowl in the fridge, so give it a quick toss or stir before serving if it's been sitting for a while.