A novel maize variety designated PHGDD and seed, plants and plant parts thereof. Methods for producing a maize plant that comprise crossing maize variety PHGDD with another maize plant. Methods for producing a maize plant containing in its genetic material one or more traits introgressed into PHGDD through backcross conversion and/or transformation, and to the maize seed, plant and plant part produced thereby. Hybrid maize seed, plant or plant part produced by crossing the variety PHGDD or a trait conversion of PHGDD with another maize variety. Inbred maize varieties derived from maize variety PHGDD, methods for producing other inbred maize varieties derived from maize variety PHGDD and the inbred maize varieties and their parts derived by the use of those methods.