The Reds had failed to score in their first two Championship matches and offered little by way of an attacking threat at the Keepmoat Stadium on Tuesday evening.
However, full-back Gunter's 31st-minute header proved enough to see off a Rovers side who remain without a point this season and have now gone 15 matches without a win.
The result was somewhat harsh on Sean O'Driscoll's men, who were the better side for large periods and created several chances, particularly in the first half.
Sam Hird replaced Richard Naylor in the centre of Doncaster's defence, while new loan signing Reece Brown - the brother of Sunderland defender Wes - was among the substitutes after joining from Manchester United earlier in the day.
Former Blackburn striker Matt Derbyshire made his Forest debut as manager McClaren made four changes from Saturday's 2-0 defeat at Millwall.
Ishmael Miller, who arrived from West Brom this week for an initial fee of £1.2million, had to settle for a place on the bench, along with highly-rated midfielder Lewis McGugan.
Rovers made a bright start and both James Coppinger and Kyle Bennett went close inside the opening four minutes. The latter's 20-yard effort appeared to have been tipped over the crossbar by visiting goalkeeper Lee Camp, but referee Andy Woolmer awarded a goal kick.
Wes Morgan hooked wide from an Andy Reid free-kick at the other end, yet it was Doncaster who were enjoying the bulk of possession, with the central midfield trio of John Oster, Simon Gillett and Giles Barnes functioning well.
Barnes forced Camp into two routine stops and the lively Bennett saw another effort fly wide as the hosts continued to look the most likely to open the scoring.
Yet it was Forest who made the breakthrough, moments after Derbyshire had seen a penalty appeal turned down, when Gunter nodded home Brendan Moloney's deep cross from the right.
Camp showed good agility to palm away an effort from Coppinger that looked destined for the corner and ensure the visitors headed into the interval with their lead intact.
Gillett had a chance to equalise early in the second half when he was picked out at the far post by Barnes, but the former Southampton man failed to connect cleanly with his volley.
Forest, who had barely threatened since their goal, remained on the back foot as the hour mark approached and McClaren reacted by bringing on American forward Robbie Findley in place of Derbyshire.
The change initially did little to improve the Reds' attacking output, although Radoslaw Majewski curled a free-kick narrowly over the bar in the 70th minute after a strong run from Findley had been unfairly halted by Hird, who received a booking.
Miller was introduced in place of David McGoldrick six minutes later, prompting large cheers from the travelling support, and the Forest faithful soon had greater reason to cheer as their side held on to claim three points, with Camp denying Gillett from a corner late on.