Catalyzes the conversion of GDP-D-mannose to GDP-4-dehydro-6-deoxy-D-mannose (PubMed:16650000). Involved in susceptibility to pore-forming crystal toxins in conjunction with bre-2, bre-3, bre-4, and bre-5 (PubMed:10924467, PubMed:17135259, PubMed:17482642). Involved in susceptibility to the nematotoxic C.cinerea galectin Cgl2, likely by contributing to the synthesis of N-glycans to which Cgl2 binds (PubMed:20062796). Has a role in determining brood size (PubMed:10924467). GDP-alpha-D-mannose = GDP-4-dehydro-alpha-D-rhamnose + H2O Optimum temperature is 30 degrees Celsius. Nucleotide-sugar biosynthesis; GDP-L-fucose biosynthesis via de novo pathway; GDP-L-fucose from GDP-alpha-D-mannose: step 1/2. Expressed throughout development, showing up-regulation between stages L2 and L4. Worms exhibit resistance to the Cry5B toxin produced by Bacillus thuringiensis. This is thought to be due to mutants having reduced population of glycolipids which are targeted by the Cry5B protein. Mutants also have reduced brood sizes at only 12% of wild-type N2. Belongs to the NAD(P)-dependent epimerase/dehydratase family. GDP-mannose 4,6-dehydratase subfamily.