Luke Chambers was sent off during the 3-0 reverse but Forest were already up against it at that point after Guly's opener.
A stroll for the Saints leaves Steve Cotterill's side in the bottom three and with no let up as they face West Ham United at Upton Park next weekend.
The manager told BBC Radio Nottingham afterwards: "You're going to be second best with 10 men against top of the table.
"The first-half performance wasn't as good as it has been and it has been pretty good for quite a while now. They were better than us.
"I thought it was a frustrating day all round for the players. I didn't think we got the rub of the green with any decisions today. Maybe we didn't earn it."