Bring a large pot of salted water to a boil then add macaroni and cook until just under al dente.
Meanwhile, melt butter in a 3 quart saucepan over medium heat. Add garlic then saute until very fragrant and just barely turning golden brown, 1 minute. Sprinkle in gluten-free flour then whisk for 1 minute. Slowly stream in milk while whisking constantly to create a smooth sauce then season with salt and pepper and turn heat up to medium-high to bring milk to a simmer — NOT a boil — stirring constantly.
Turn heat down to medium then stir constantly until sauce has thickened, 5 minutes. Remove saucepan from heat then add shredded cheese in two batches, stirring until very smooth before adding the second batch. Taste then add salt and/or pepper if necessary.
Drain pasta then add to the cheese sauce and stir to combine. Add tomatoes and basil then stir to combine. Serve immediately.