Video replays showed the 21-year-old stamping on Leeds winger Watt as Forest prepared to defend a corner 10 minutes from time.
The clash provoked a prolonged scuffle involving no fewer than 20 players.
Grayson said: "I don't know how they (Bates and his assistants) have come to the conclusion that it's a yellow card for Gunter because it
was a blatant stamp.
"I can't see how it's anything less than a sending off and I think the referee made the wrong decision.
"I've seen the DVD and he also gave Sanchez a dig in the back before he
stood on him. Gunter's a lucky boy and how they've decided it's two
yellow cards I've no idea.
"Sanchez wasn't even looking in Gunter's direction and he accidentally stood on his foot. That's fair enough because it was not deliberate. I believe Gunter's stamp was."