Preheat grill for low heat and lightly oil the grate.
Place beer can on a disposable aluminum baking sheet and set chicken upright onto the can, inserting can into cavity. Mix ranch salad dressing, onion, garlic salt, salt, and ground black pepper in a bowl. Paint the outside of chicken with ranch dressing mixture.
Place chicken on the baking sheet on the preheated grill. As the chicken roasts, paint the skin several times with the ranch dressing mixture. Cook until an instant-read thermometer inserted into the thickest part of a thigh, not touching bone, reads 180 degrees F (80 degrees C), about 3 hours.