In a small bowl, mash together the butter, lemon zest, lemon juice, the 1/4 teaspoon pepper, and the garlic powder. Set aside (or chill if not using immediately).
Prepare the grill for medium-high direct heat and lightly oil the grate (or preheat a broiler). Brush oil on both sides of tuna steaks. Season the tuno on both sides with salt and additional pepper. Grill or broil for 3 to 4 minutes per side for medium rare, turning once.
Transfer tuna steaks to a serving platter; top with the butter mixture.