Generating the mesh using quadrilaterals

Select Mesh->Element type->Quadrilateral. Select surfaces number 24 and 12 and press ESC.
Select Mesh->Generate mesh.
A window comes up asking whether the previous mesh should be eliminated. Click Yes.
Another window appears in which the desired element size can be entered. Leave the default value unaltered and click OK. The result will be the mesh illustrated, where it can be appreciated the quadrilateral mesh of both surfaces.

An unstructured mesh generated using quadrilaterals.

GiD user manual