Adjust oven rack to lowest position, place rimmed baking sheet on rack.
Place grapefruit segments in fine-mesh strainer set over medium bowl and drain for 15 minutes.
Meanwhile, make several shallow slashes about an inch apart along skin side of each salmon fillet, being careful not to cut into flesh.
Pat salmon dry with paper towels. Rub fillets evenly with oil and season liberally with salt and pepper.
Reduce oven temperate to 275* degrees and remove baking sheet.
Carefully place salmon skin-side down on baking sheet. Roast until centers of thickest part of fillets are still translucent when cut into with paring knife or instant-read thermometer inserted in thickest part of fillets registers 125* degrees, 9 to 13 minutes.
Meanwhile, pour off all but 1 tablespoon grapefruit juice from bowl; whisk in grapefruit segments, shallot, basil, lemon juice, and oil.
Stir in grapefruit and season to taste with salt and pepper.
Remove salmon and top with relish; serve warm or cold.