Fiberglass mesh typically consists of fiberglass strands woven together to form a mesh fabric. The "45 gsm" you mentioned refers to the weight of the mesh in grams per square meter. This indicates how much the material weighs per unit area.
GSM stands for grams per square meter, and it's a common measurement used to describe the weight or density of fabrics and other materials. In the case of fiberglass mesh, a 45 gsm weight suggests that the mesh is relatively lightweight.
As for the content, fiberglass mesh is primarily made of glass fibers. These fibers are often coated with a material such as resin to enhance durability and resistance to weathering. However, the specific composition may vary depending on the manufacturer and intended use of the fiberglass mesh.