merging surfaces for structural mesh
 

Dear Users,
I have encountered a very basic problem considering GMSH. I am tryign to learn how to use this softwere however I cannot find solution to my issue. I have a geometry STP file with a complex geometry which I would like to mesh. The geometry is divided in such a way that the structural mesh should be generated.

The problematic part of the body is presented in the figure below. When the geometry is imported into the GMSH an additional line is generated. This line splits the surface into two smaller ones. The problem is I cannot merge the surfaces using boolean options. In the result it is impossible to generate structural mesh on two of the surfaces since there are 5 vertexes instead of 4 (two lying on the bottom and three on the top).

I am aware that this is a very simple case but could you help mi with that?

Best regards