Sub Berkeley, who was recalled from a loan spell at Conference North Club Workington just a week ago, pounced to head in from a corner in the final minute.
Despite the efforts of a defender who hooked the ball away off the line, it crashed into the roof of the net to leave the 18-year-old with a huge grin on his face.
Gretna went in front in the opening minute when James Grady's shot was deflected into the path of Jamie McQuilken, who fired in at the back post from close range.
In the 12th minute Peterhead only half cleared a corner and Gretna new boy Scott Paterson fired in a fierce volley from 16 yards that keeper Paul Mathers parried away spectacularly at full stretch.
But the Blue Toon levelled on the half hour when Paterson was booked for a deliberate hand-ball in the dangerous position on the edge of the box, and Gretna were made to pay when Douglas Cameron sent the resultant free-kick into the back of the net.
Gretna went 2-1 up in the 69th minute when Grady slipped in substitute Dene Shields, who sent a low left-foot drive crashing inside the near post.
And Berkeley, who had only been on the pitch eight minutes, wrapped things up in the dying seconds when Gavin Skelton whipped in a corner and he flicked in a header that deflected off a defender and was rifled into the roof of the net by another.