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The CPI reflects prices of goods and services in the standard market basket. The GDP deflator reflects prices of goods and services across the entire economy. Each of them can be used to turn current/nominal dollars into constant dollar amounts as of some base time period, but that's not what multipliers are typically about.
Multipliers typically describe the final effect on GDP (total economic activity) of certain policy options. Increases in UI or SNAP benefits for instance have relatively high multipliers because the benefits are spent quickly by the recipients and are spent by handing the money over to others who are apt to spend it quickly. More turnover of dollars equals more economic activity equals higher GDP. Things like accelerated depreciation and general business tax cuts have relatively low multipliers. They do not tend to generate much additional economic activity.