It's one of the most discussed economic numbers on the face of the planet

At FRED, search for gross federal debt. 32 series in the search results.

Select the Frequency tag for Quarterly data. The list is reduced to 16 series.

Select the Geographies tag for United States of America. Seven series remain. Six of these are "Percent of GDP, Quarterly, Seasonally Adjusted". The odd man out is "Billions of Dollars, Quarterly, Seasonally Adjusted". That's what I want, billions. That series is Federal Government Debt: Gross (DISCONTINUED), for the period Q1 1953 to Q4 1998. Nothing current.

If you want to know Gross Federal Debt in billions, quarterly, don't go to FRED.