There are several examples in the glpsol examples collection

graph.mod shows the fanciest input format

color.mod is more interesting from a modelling point of view

vertex-cover.mod uses the graph from graph.mod for a vertex cover example.

independent-set.mod uses the graph from graph.mod for an independent-set example. In this example the difference between the LP relaxation and the integer optimal is not too bad, compared to the worst case.