The present study focuses on the experimental investigation on the optimal pilot injection strategy under GCI combustion mode in a multi-cylinder heavy-duty diesel engine. Three experiments were conducted at a highspeed high-load operating point with different operating parameters and emission targets, namely engine-out NOx target, fuel injection pressure, and main injection timing. The engine-out NOx targets were set to 5.0 g/kWh and 1.5 g/kWh, and the gasoline injection pressures were set to 100 MPa and 140 MPa. These high and low values represent different requirements of SCR efficiency and practical capability of fuel supply system when addressing the hypothetical future tailpipe NOx limit of 0.02 g/hp-hr (0.027 g/kWh). The results show that the use of optimized pilot injection always achieves lower pressure rise rate and soot emissions than the single injection baseline. The pilot gasoline fuel with low injection pressure is more ignitable than that with high injection pressures, hence a distinct heat release spike usually occurs for a pilot injection. The optimal pilot mass should be increased for a higher fuel injection pressure because the pilot fuel stratification level decreases. A relatively late pilot timing is preferable for the early main injection timing. For the late main injection timing, however, a relatively early pilot timing with large pilot mass is preferable and brings about a distinct two stage high temperature heat release, which can reduce the fuel consumption and soot emissions simultaneously. The two stage split combustion process obtained by double injection with retard combustion phasing can be considered to be an important way to alleviate the requirement of GCI fuel system.