The GDP Deflator is mathematically classified as a Laspeyres index because it uses a fixed basket of goods from the base year to measure price changes in the current year.

The GDP Deflator is actually a Paasche index because it uses the quantities of the current year as its weights, reflecting the actual basket of goods and services produced in that specific period. In contrast, the CPI is typically a Laspeyres index, which uses a fixed, historical base-year basket, which can lead to an overestimation of inflation due to the substitution bias.