Guppy put Leeds ahead when he rose at the far post to head home Frazer Richardson's cross from the right.
But Forest were handed a lifeline when Guppy needlessly fouled Paul Evans in the area and Andy Reid stepped up to convert the penalty.
Leeds pushed forward and Julian Joachim forced a great save from Paul Gerrard before Brian Deane's downward header bounced over - but Forest held on.
Forest boss Joe Kinnear on his side's performance:
"We remain undefeated and worked hard for the result.
"We chased the game and got rewarded for a little bit of bravery.
"We were throwing people forward, putting people under pressure and Andy Reid tucked the penalty away nicely."
Leeds: Sullivan, Kelly, Butler, Carlisle, Crainey, Richardson,
Wright, Guppy (Walton 79), Pugh, Joachim (McMaster 80),
Deane (Ricketts 82).
Subs Not Used: Carson, Kilgallon.
Goals: Guppy 25.
Nottm Forest: Gerrard, Louis-Jean, Morgan, Hjelde, Rogers,
Impey, Evans, Jess (King 73), Reid, Johnson, Taylor.
Subs Not Used: Roche, Doig, Commons, Robertson.
Booked: Evans, Reid, Hjelde.
Goals: Reid 78 pen.
Ref: M Pike (Cumbria).