This guide presents the analytical underpinnings and a manual for the Public Debt Dynamics Tool with Gross Financing Needs (DDT_GFN), an extension of the Public Debt Dynamics Tool (DDT). The DDT projects public debt as percent of GDP under a baseline and alternative scenarios. The DDT_GFN presents debt-to-GDP projections and the corresponding public gross financing needs (GFN)-to-GDP projections for all DDT scenarios, including fan charts for both public debt and GFN. The DDT_GFN also computes fiscal adjustment paths for a user defined GFN target and adjustment period. Twelve often publicly available macrofiscal variables are needed to perform a rich analysis of public debt and GFN dynamics. The DDT_GFN was developed by the IMF Institute for Capacity Development to be used for its capacity development work on macroeconomic frameworks for forecasting and policy analysis.