She first campaigned for the Liberal Party in the 1984 federal election, and later supported Jean Chrétien for the Liberal Party leadership in 1990 ("Calgary Herald", 10 July 1989). She won the Calgary West Liberal nomination for the 1993 election on November 24, 1992, defeating 1988 candidate John Phillips by 182 votes to 158. Gainer supported official recognition for same-sex marriages during the 1993 campaign, several years before the issue became prominent in Canadian politics ("CH", 6 October 1993). She finished second against Reform Party candidate Stephen Harper, and was later appointed to the National Forum on Health.