Base isolation is a widely employed technique in earthquake engineering aimed at safeguarding structures from seismic damage by isolating them from the ground through specialized isolators. These isolators experience significant deformations during earthquakes, dissipating a substantial portion of the seismic energy and enabling the superstructure to behave as a linear, rigid body. However, this increased deformation necessitates specific demands on isolator dimensions and seismic gap, which can be challenging to accommodate utilities near the isolation layer. To address these challenges, additional devices can be introduced into base-isolated structures (BISs) to mitigate lateral displacements. One such device is an inerter, which is a passive mechanical device, and when paired with base isolators, it enhances seismic energy dissipation and reduces structural response, improving earthquake resilience. While numerous studies have validated the enhanced seismic performance of BISs equipped with inerters, limited experimental studies have been conducted due to the challenges involved in constructing BISs for experimental purposes. To address this gap, an experimental study through real-time hybrid simulation (RTHS) was conducted to evaluate the performance of inerters in reducing seismic demands on a BIS. A nine-story steel frame benchmark building is retrofitted to become a BIS with lead rubber bearings (LRBs), which serves as the numerical substructure. The inerter device paired with the LRBs is the gyro-mass damper (GMD) acting as the experimental substructure. The results obtained through RTHSs demonstrate that the incorporation of GMDs enhances the seismic performance of the BIS by reducing maximum story displacement, interstory drift ratios, and base shear.