import bpy
#Change gab
gab = 1
objs = bpy.context.selected_objects
box = []
for i in range(len(objs)):
    objs[i].location = (0,0,0)
    box.append(objs[i].dimensions[2])
    if (i == 0):
        box[0] = box[0]/2
    else:
        objs[i].location[2] = (sum(box[:i])+objs[i].dimensions[2]/2)+gab*i