A novel cyanide-free gold electroplating bath was developed with 2-hydroxyphosphonoacetic acid (HPAA) as the core complexing agent in this work. Scanning electron microscopy (SEM) observations demonstrate that the obtained gold electrodeposits possess a smooth and compact surface morphology. The crystal structure of the gold electrodeposits was characterized via X-ray diffraction (XRD), and the coating–substrate adhesion was systematically evaluated through scratch tests. Molecular dynamics (MD) simulations were performed to investigate the adsorption interaction between HPAA and metal (Au/Ni) surfaces. The MD simulation results show that all the studied phosphonate-containing derivatives can strongly adsorb on the gold surface and exert a significant inhibitory effect on the electroreduction of gold ions during electrodeposition. Cyclic voltammetry (CV) and other electrochemical tests reveal that the cathodic reduction peak potential of gold shifts significantly negatively after the addition of phosphonate-based organic additives, which effectively enhances the cathodic polarization of gold deposition, delays the gold nucleation rate, and refines the grain size of electrodeposits, ultimately yielding gold electrodeposits with a denser and smoother surface. Owing to its environmental benignity, excellent process stability and superior coating performance, this cyanide-free gold electroplating system exhibits broad application prospects in the field of modern green surface engineering.