The fate of sulfur and conversion of metals during the co-gasification of municipal solid waste (MSW) and gypsum is examined here using aspen plus combined with Thermo-Calc for the process model development. The effect of air ratio, temperature, and MSW-to-gypsum feed mass ratio on the syngas evolution, sulfur transformation, and mineral speciation behavior is investigated. The results showed prevention of gypsum sulfur transformation to sulfur dioxide at temperatures below 1050 °C, air ratio < 0.4, and MSW-to-CaSO4 feed mass ratio < 33 wt%. Approximately 90 wt% of feed was transformed into gas products comprising 22% CO and 19% H2. At approximately 900 °C, major minerals formed were CaS (alabandite), melilite, anorthite, rankinite, nepheline, and wollastonite. Melilite, a calcium silicate of aluminum and magnesium, dominated over all other silicates. At temperatures >1000 °C, these minerals transformed into a more stable calcium orthosilicate (CaSiO4) and molten oxysulfide. At temperatures higher than 1200 °C, all metals in MSW were transformed into molten oxides. The results show that syngas and minerals can be recovered during the co-gasification of MSW and gypsum to directly reveal the synergetic benefits of co-processing MSW and gypsum low-value waste materials.