var doc = app.documents[0];
var placeGun = app.documents[0].placeGuns;
var placeGunContentsFirstLevel = placeGun.pageItems.everyItem().getElements();

// Detect the default object style:
var tempRect = doc.rectangles.add({ geometricBounds : [0,0,"50mm","50mm"] });
var defaultObjectStyle = tempRect.appliedObjectStyle;
tempRect.remove();

// Apply the detected default object style to all graphic frames in the place gun that contain graphics:

for( var n=0; n<placeGunContentsFirstLevel.length; n++ )
{
	var allGraphics = placeGunContentsFirstLevel[n].allGraphics;
	for( var g=0; g<allGraphics.length; g++ )
	{
		allGraphics[g].parent.appliedObjectStyle = defaultObjectStyle ;
	};
};