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.