GAM Backfill Revenue is a Metric in Google Analytics under the DoubleClick for Publishers Backfill section
GAM Backfill Revenue Definition: The total estimated revenue from backfill ads. If AdSense and Ad Exchange are both providing backfill ads, this metric is the sum of the two backfill accounts (DFP linking enabled and Hide DFP Revenue not enabled).
Data Analytics & Insights Google Analytics Metrics GAM Backfill Revenue