The Gateway Festival Orchestra will perform music of Jean Sibelius, Launy Grøndahl and Edvard Grieg at 7:30 p.m. Sunday, July 15, as part of its 2018 season of free summer concerts. Also on the program will be music from “Mamma Mia,” based on songs by the pop group ABBA. The concert series will continue July 22 and 29.