Catalysis of the reaction: polysaccharides containing a terminal alpha-L-guluronate group = oligosaccharides with 4-deoxy-alpha-L-erythro-hex-4-enuronosyl end. This reaction is the eliminative cleavage of polysaccharides containing a terminal a-L-guluronate group, to give oligopolysaccharides with 4-deoxy-a-L-erythro-hex-4-enuronosyl groups at their nonreducing ends.