Set up a grill for two zone cooking, with direct and indirect cooking. Place the butter, garlic, olive oil, and rub in an aluminum pan on the indirect side of the grill to melt.
Spray the ribeye with the duck fat spray and season liberally with the rub.
Place the ribeye directly over high heat for 2 minutes to sear. Rotate the steak a quarter turn and sear for another 2 minutes. Flip the steak over and sear for another 2 minutes. Rotate the steak a quarter turn and sear for another 2 minutes.
Move the steak to the indirect cooking zone and cook to your desired temperature, 20 to 30 minutes for rare,125 to 130 degrees F (50 to 55 degrees C). Baste with the butter mixture every 5 to 10 minutes while cooking.
When the desired temperature is reached, remove the steak to a cutting board and tent it with aluminum foil to rest for at least 10 minutes before slicing.