GAM Backfill eCPM is a Metric in Google Analytics under the DoubleClick for Publishers Backfill section
GAM Backfill eCPM Definition: The effective cost per thousand pageviews. It is the DFP Backfill Revenue per 1,000 pageviews. If both AdSense and Ad Exchange are 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 eCPM