Look at the gray roof on a clear day. This will give you the most color information and allow you to see the subtle tones better.
Assess whether the gray is truly neutral by holding the 18 percent gray card, with the sun at your back, so it obscures your view of the roof. Look back and forth at the card and the roof. The card is a pure, neutral gray in an exact mid-tone (halfway between black and white). If the roof looks bluish or purplish, it is cool. If it looks reddish or yellowish, it is warm. If you cannot detect it leaning either way, it is probably mostly neutral.
Select a warm color to match a warm gray. This means yellow, green-yellow, orange, red or variants thereof. Select a cool color to match the roof if it's cool gray. This means blue, blue-green, purple or variants thereof. For a neutral gray roof, any color will match.