The High Energy cosmic-Radiation Detection (HERD) facility has been proposed as one of the main scientific payloads on-board the China Space Station. HERD is expected to be installed around 2027 and to operate for at least 10 years. The major scientific goals of the mission are the study of cosmic rays' energy spectra and composition up to the PeV range, the indirect search of dark matter particles via annihilation/decay, and observation of the high-energy gamma-ray sky above $\sim 100\,\mathrm{MeV}$. To accomplish the broad scientific case, HERD is designed as a large acceptance telescope: a central 3D imaging calorimeter (CALO) is surrounded on the top and the four lateral sides by scintillating fiber tracking detectors (FIT), plastic scintillator detectors (PSD) and silicon charge detectors (SCD). In order to maximize the performance for gamma rays studies, an ultra-low-energy gamma-ray (ULEG, $E>100\,\mathrm{MeV}$) advanced trigger system, based on the fast combination of 3-in-a-row patterns in FIT and the PSD veto, has been designed. In this work we characterize the performance of the HERD-ULEG trigger in terms of gamma-ray effective area, angular resolution and sensitivity. We also present the design, performance and optimization of the gamma-ray trigger system based on both software simulations and preliminary hardware validations.