Background/Objectives: The GARP transcription factor superfamily is crucial for plant growth, development, and stress responses. This study systematically identified and analyzed the GARP family genes in Populus deltoides to explore their roles in plant development and abiotic stress responses. Methods: A total of 58 PdGARP genes were identified using bioinformatics tools. Their physicochemical properties, genomic locations, conserved motifs, gene structures, and phylogenetic relationships were analyzed. Expression patterns under phosphorus and nitrogen deficiency, as well as tissue-specific expression, were investigated using RT-qPCR. Transgenic RNAi lines were generated to validate the function of GLK genes in chlorophyll biosynthesis. Results: The 58 PdGARP genes were classified into five subfamilies based on their evolutionary relationships and protein sequence similarity. Segmental duplication was found to be the primary driver of the PdGARP family’s expansion. Cis-regulatory elements (CREs) related to light, hormones, and abiotic stresses were identified in the promoters of PdGARP genes. Differential expression patterns were observed for NIGT1/HRS1/HHO and PHR/PHL subfamily members under phosphorus and nitrogen deficiency, indicating their involvement in stress responses. KAN subfamily members exhibited tissue-specific expression, particularly in leaves. Structural analysis of the GLK subfamily revealed conserved α-helices, extended chains, and irregular coils. Transgenic RNAi lines targeting GLK genes showed significant reductions in chlorophyll and carotenoid content. Conclusions: This study provides a comprehensive analysis of the GARP transcription factor superfamily in P. deltoides, highlighting their potential roles in nutrient signaling and stress response pathways. The findings lay the foundation for further functional studies of PdGARP genes and their application in stress-resistant breeding of poplar.