Brian Davis shot a four-under 68 to finish one shot further back at five- under-par 211 alongside David Lynn.
Struver enjoyed a one-shot edge over Diego Borrego at the start of the day but his round immediately came apart at the seams with back-to-back bogeys from the first.
The German dropped another shot with a bogey at the sixth and fell to four- under with a bogey at the par-four ninth.
Struver recovered with some fine irons on the inward nine and picked up a birdie at the 11th. He chipped within three feet of the cup for birdie at the 15th and hit his approach to the same distance at the following hole to climb to seven-under and regain sole possession of first place.
However, the 34-year-old bogeyed the closing par-four to fall back into a tie with Gilford and Tunnicliff.
'I am pleased with the way I came back,' said Struver. 'It could have got worse for me after my front nine but I saw the guys up front weren't making that big a move. Even though the back nine is perhaps easier, you still have to hit good shots to make birdies and I did that so I was pleased.'
Gilford had the low round of the day with a five-under 67. At the 15th, the Englishman's approach shot nearly found the hole. He tapped in for birdie and drained an eight-footer at the 17th to finish at minus-six.
Tunnicliff briefly held a three-shot lead over the rest of the field after moving to eight-under thanks to a brilliant front nine over De Vere Slaley Hall Golf Club.
The 33-year-old, who is seeking his first win on the European Tour, collected three straight birdies from the second. He then birdied the sixth and the seventh before rolling in a long birdie putt at the eight to make the turn with a 30.
Tunnicliff found trouble on the back side with a three-putt bogey at the 13th and dropped another shot at the last for a round of 68.
'It was a tough windy day so it was inevitable I would do that coming home,' said Tunnicliff. 'I didn't want to make mistakes.'
Rolf Muntz, Gary Murphy, Nicolas Vanhootegem and Roger Wessels are knotted at four-under-par 212.
Malcolm Mackenzie, Brett Rumford and Paul Casey were one shot further back at three-under-par 213.
Defending champion Andrew Coltart was five shots off the pace after he carded a round of two-under 70. He finished at one-under-par 215 alongside Jonathan Lomas, Garry Houston and Borrego.
Full field scores from The Great North Open