As with all BEA aggregate price indexes, the GDP implicit price deflator is a Fisher chain price index with the average change in prices weighted by the composition of products and service bundles at the beginning and end of each quarter (Chapter 4, NIPA Handbook).
To compare the amount of societal resources expended in different periods (i.e., opportunity cost), the preferred index is the GDP implicit price deflator, which captures overall economic activity rather than consumer spending.