Gypsum boards formed from synthetic gypsum and other gypsum sources having high chloride salt concentrations. Gypsum boards may include a board core including set gypsum between a front cover sheet and a back cover sheet. A total concentration of the chloride anion in the board core ranges from about 150 to 4000 parts by weight (pbw) chloride, typically about 300 to about 3000 pbw, more typically from about 400 to about 2000 pbw, further typically from about 1000 to about 2000 pbw, and further typically from about 600 to about 1000 pbw, per 1,000,000 pbw calcium sulfate dihydrate. A polymer layer contacts an inner surface of the back cover sheet. A densified gypsum layer contacts an inner surface of the front cover sheet. Methods of making the gypsum board, and a wall system for employing the gypsum boards, are also provided.