Nanohybrids, comprising gold nanoparticles (Au NPs) and polymer NPs have attracted significant attention in recent years due to their excellent physical and chemical properties. In this work, a novel kind of nanohybrids based on Au NPs and biocompatible poly(γ-glutamic acid)-graft-3,4-dihydroxy-L-phenylalanine (γ-PGA-g-DA) NPs was conveniently achieved through a green and facile one-pot strategy. By simply adding HAuCl₄·3H₂O to the aqueous solution of γ-PGA-g-DA copolymer, the decrease of the pH value induced the self-assembly of γ-PGA-g-DA into NPs, during which Au³⁺ was adsorbed in the in situ formed NPs and spontaneously reduced to Au NPs by the dopamine moieties of γ-PGA-g-DA, leading to the formation of Au@γ-PGA-g-DA nanohybrids. In the reaction process, γ-PGA-g-DA was used as scaffold and reductant simultaneously, avoiding the use of extra toxic reducing reagents. Interestingly, the particle size and clustering of Au NPs in Au@γ-PGA-g-DA nanohybrids were varied with the loadings of HAuCl₄·3H₂O. This one-pot preparation process is very simple, fast, and completely based on the principles of green chemistry. Furthermore, the resultant biocompatible Au@γ-PGA-g-DA nanohybrids were used to detect L-tryptophan and exhibited a good analytical performance. The novel Au@γ-PGA-g-DA nanohybrids are a promising system for detecting the amino acids and biological species.