<p>Gray cast iron is extensively utilized in various industries, such as automotive brake discs and machine tool beds, owing to its exceptional wear resistance, thermal conductivity, and moderate strength. Although the laser remelting of ductile iron has reached a mature stage, the underlying mechanism responsible for pore formation during the laser remelting process of gray cast iron remains unclear. Consequently, the previous approaches have encountered limitations in achieving a non-porous remelting layer, thereby restricting the practical application of gray cast iron. The present study applied optical microscope and oxygen nitrogen hydrogen analyzer to compare the microstructure and content of oxygen, nitrogen, and hydrogen in QT400 ductile iron and HT250 gray cast iron prepared under identical casting process conditions. This study promoted the design and development of an adjustable nozzle to achieve laser remelting under hypoxic conditions. A comparison was made between the porosity of the laser remelted layer in ductile iron and gray cast iron under air/oxygen-deficient conditions. Results demonstrated the absence of pores in QT400 ductile cast iron regardless of the processing condition employed, whereas pores were consistently observed in HT250 gray cast iron irrespective of its processing conditions. Based on the initial conditions and experimental outcomes of QT400 ductile iron and HT250 gray iron, it could be inferred that the increased porosity observed in HT250 gray iron following laser remelting was attributable to the morphology of its graphite. In conjunction with this mechanism, a pore-free laser remelted layer was fabricated on HT250 gray cast iron through the composite process of vacuum heat treatment and laser remelting with an air curtain.</p>