Plastic production has grown faster than any other material in recent decades. As a consequence, a large amount of plastic waste is accumulating, which has a great impact on terrestrial and marine ecosystems. The aims of this research are to study the water resistance properties of new eco-friendly gypsum composites made with Low-Density Polyethylene (LDPE) waste additions in granular form. Three percentages of LDPE additions by weight have been used as partial replacement of the original gypsum material: 2.5%, 5.0% and 7.5%; the diameters of the LDPE aggregates were between 1 and 2 mm. The results show that the addition of these recycled raw materials reduces the total water absorption of the gypsum composites. On the other hand, durability tests have been carried out against the repeated action of wet chamber cycles and water–stove cycles. After carrying out these accelerated aging tests, it was concluded that all the composites produced in this research exceeded the minimum flexural and compressive strengths recommended by the EN 13279-2 standard. In view of the above, the gypsum composite materials produced are a sustainable alternative for recovering and revaluing plastic waste. In this way, granular LDPE waste shows its feasibility to be incorporated as a secondary raw material in the development of new precast construction products made under circular economy criteria.