Midfielder Steve Tosh hit a superb double to clinch the Second Division championship for Gretna with six games to play.
Tosh hammered in both goals from corners as Gretna were made to work hard by relegation-haunted Alloa, who dominated for long stages.
The Second Division basement club took a shock lead in the second minute when Gretna didn't deal with a high ball and James Stevenson crashed an angled shot across David Mathieson into the bottom corner from 12 yards.
It almost got worse for Gretna 60 seconds later when Stevenson thrashed a shot from the edge of the box just past the angle of the post and bar.
Four minutes later Robert Sloan crossed to the near post, and Paul McLeod's drive was brilliantly turned over by David Mathieson as Alloa piled on the pressure.
In the 29th minute John O'Neil curled in a free-kick that Kenny Deuchar headed just wide, but Gretna's attack was lacking its usual punch with Alloa quick into tackles and moving the ball smartly around the park.
Gretna levelled on the stroke of half-time when Derek Townsley headed back a Gavin Skelton corner into the path of Tosh, who unleashed a 20-yard drive into the corner of the net.
Tosh struck again in the 67th minute when Townsley set him up once more from an O'Neil corner, and he let fly with a low left-foot drive from 20-yards that crashed into the net.
But Alloa came back and Sloan set up John Grant, but Mathieson finger-tipped his drive over the bar.
The Wasps forced a number of corners as they continued to battle hard for Second Division survival, and in the final minute Michael Bolochoweckyj's powerful downward header from a corner was cleared off the line at the back post by Gavin Skelton and Gretna held on to take the three points.