According to the source, gross output is a measure of an industry's sales or receipts, which can include sales to final users in the economy (GDP) or sales to other industries (intermediate inputs).
Source: US. Bureau of Economic Analysis
Release: Gross Domestic Product by Industry
US. Bureau of Economic Analysis, Gross Output of Private Industries: Finance, Insurance, Real Estate, Rental, and Leasing (Chain-Type Price Index) [GOPIFIRL], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/GOPIFIRL/, July 7, 2015.