lc = 1.0;
L  = 3.0;
x_P = 0.0;
y_P = 0.0;
z_P = 0.0;
Point(1) = {x_P, y_P, z_P, lc}; // center of the cube

// Points on a surface 
Point(2) = {x_P - L/2, y_P - L/2, 0, lc};
Point(3) = {x_P + L/2, y_P - L/2, 0, lc};
Point(4) = {x_P + L/2, y_P + L/2, 0, lc};
Point(5) = {x_P - L/2, y_P + L/2, 0, lc};

// Line between points
Line(1) = {2, 3};
Line(2) = {3, 4};
Line(3) = {4, 5};
Line(4) = {5, 2};

// Generate surface
Line Loop(5) = {4, 1, 2, 3};
Plane Surface(6) = {5};

// Extrude surface to get the cube
Extrude {0, 0, L} {
  Surface{6};
}

Physical Surface(3) = { 6, 15, 23, 19, 27, 28 };
Physical Volume(1) = { 1 };