PROVIDENCE — Gateway Healthcare has merged two residential treatment programs to form a 16-bed program to treat adolescents with mental health and substance use problems.The new Caritas ARTS program, located at 70 East Ave., Cranston, was formed out of the merger of the Acute Residential Treatment Services (ARTS) in Johnston, a coed eight-bed facility, and the 16-bed Caritas program in Cranston, for adolescent girls.