Preheat grill to hot.
Sprinkle eggplant, zucchini and squash with salt and pepper.
Grill vegetables until done, about 2 minutes for zucchini and squash and 4 - 5 minutes for eggplant. Remove and place on platters. Set aside to cool.
Preheat oven to 350°F.
In an oven safe pan, arrange eggplant slices on the bottom, closely next to each other, making sure to try and cover up any possible holes there may be.
Top with 1/3 of the tomato sauce and 1/3 of the cheese.
Arrange zucchini on top in the same way as the eggplant.
Top with 1/3 of the tomato sauce and 1/3 of the cheese.
Arrange summer squash on top in the same way as the eggplant and zucchini.
Top with remaining tomato sauce and cheese.
Sprinkle with the garlic, basil and red pepper flakes.
Cover with aluminum foil and place in preheated oven. Bake until cheese is melted and the lasagna is heated all the way through, about 35 minutes.
Remove aluminum foil and place back in oven. Bake another 10 minutes.
Remove from oven and allow to sit for 5 - 10 minutes.
Cut and serve.