Run over grenade, pick up grenade. Once the grenade is selected via the toss gren (which in effect brings grenade out of your slot an into your hands, you would need to hit toss green to reholster grenade. And using the attack button to throw also prohibits the use of the gun. Once ready to throw consult the book
" And the Lord spake, saying, 'First shalt thou take out the Holy Pin. Then, shalt thou count to three. No more. No less. Three shalt be the number thou shalt count, and the number of the counting shall be three. Four shalt thou not count, nor either count thou two, excepting that thou then proceed to three. Five is right out. Once the number three, being the third number, be reached, then, lobbest thou thy Holy Hand Grenade of Antioch towards thy foe, who, being naughty in My sight, shall snuff it.' "
Five should be thy count.