The square lid consists of two triangular parts, so that the opening is done by lifting one of the parts, and the covers usually do not come separately but in groups of 2 or 3 that form a rectangle.
The illustration on each of the parts of the cover looks like a cable of fibers, according to which it seems that it is a communication cover
GS - Spheroidal graphite cast iron - a special cast iron in which the crystallization of carbon has been slowed down, which makes it close to steel - also called ductile iron (Thanks to Vuori Susi for the clarification)