Gibberellins (GAs) promote flowering in the forcing-cultured tree peony (Paeonia suffruticosa), however, the mechanism of regulating flowering is not fully understood. In this study, exogenous GA3 was applied to five-year-old Luoyang Hong plants to explore responses in terms of endogenous hormones, flowering quality, and the hormone- and flowering-associated gene expression. Exogenous GA3 application significantly promoted flower bud development and new branch growth, as well as improved flowering quality. Exogenous GA3 application also stimulated the synthesis of endogenous GA3 and indole-3-acetic acid (IAA) but reduced abscisic acid (ABA) levels. To further elucidate the regulatory mechanism, eight genes for GA biosynthesis and signaling, including PsCPS, PsKS, PsGA3ox, PsGA2ox, PsGID1b, PsGID1c, PsDELLA, and PsGID2 were cloned for the first time, and sequence analysis was also performed. The results suggested that all the cloned genes have conserved structure as each homologous gene reported in the other species. Phylogenetic trees constructed by the each cloned gene showed that the phylogenetic evolutionary relationship of P. suffruticosa was closely related to Vitis vinifera. The expression patterns of the above genes, and genes for ABA and IAA biosynthetic and signaling, and the flowering time were also investigated. Most of the above genes showed higher expression in the control buds than those in the GA3 treated buds at six developmental stages, whereas the expression levels of PsSOC1 and PsSPL9 were up-regulated by GA3 treatment. The results also showed that the GA-biosynthetic and signaling pathways are conserved in tree peony, and the PsCPS, PsGA3ox, PsGA2ox, PsGID1, PsDELLA, and PsGID2 genes are necessary for feedback regulation of GAs. Furthermore, hormone changes promoted PsSOC1 and PsSPL9 expression, and repressed PsSVP expression, which contributed to the improvement flowering quality in tree peony of forcing culture.