The present study shows the distinctive role of first injection proportion and fuel ignition quality in controlling the first-injection mixture homogeneity and the second-injection charge premixing, respectively, in a single-cylinder automotive-size gasoline compression ignition (GCI) engine running on a double injection mode. The engine was operated at 2000 rpm and the indicated mean effective pressure (IMEP) of about 940 kPa. From many previous studies on homogeneous charge compression ignition (HCCI) engines, it is well known that the increased mixture homogeneity can reduce engine-out emissions of particulates and nitrogen oxides (NOx) while increased unburnt hydrocarbon (uHC) and carbon monoxide (CO) emissions become an issue. Also, many diesel combustion studies showed that the increased charge premixing causes the higher initial rate of heat release and thereby increasing engine efficiency at the expense of NOx emissions. In partially premixed charge GCI combustion engines implementing a near-BDC first injection and a near-TDC second injection, however, it is unclear how the overall mixture homogeneity and premixedness impact engine efficiency and engine-out emissions. In this study, both the first injection proportion and fuel ignition quality were varied considering their expected influence on premixed charge preparation. Specifically, conventional gasoline fuels with 91, 95, and 98 research octane number (RON) were tested for GCI combustion for various first injection proportions of 20-50%. For fixed combustion phasing conditions realised by the variations of the second injection timing, the results show that the increased first injection proportion causes a more stretched profile of the apparent heat release rate (aHRR) with a lower peak value due to the increased mixture homogeneity of the first-injection charge. This leads to increased engine efficiency as the heat release occurs for a longer period of time while the more homogeneous mixture causes reduced NOx emissions and combustion-induced noise. The increased mixture homogeneity, however, leads to increased uHC and CO emissions. In comparison, the decreased fuel ignition quality (or increased RON) at fixed mixture homogeneity of the first-injection charge results in higher aHRR and thereby increasing the engine efficiency, which is due to the increased charge premixedness of the second-injection fuel. Consequently, the NOx emissions and combustion-induced noise increase while the uHC/CO/smoke emissions become lower. Due to these compounding effects of first-injection mixture homogeneity and second-injection charge premixedness, it was found that the optimised GCI engine efficiency and emissions, for selected operating conditions and given engine geometry, are achieved at 40% first injection proportion and 95 RON fuel.