Gasoline compression ignition (GCI) engine technology has become one of the promising alternative solutions to achieve better fuel economy and meet emission requirement. Higher reactivity gasoline-like fuels are more desirable in GCI engines. This study investigates the soot processes under autoignition combustion of high-reactivity gasoline (HRG) with an outwardly opening piezo gasoline direct injection (GDI) fuel injector. HRG fuels are mixtures of refinery streams with RON of 50–80 and they can potentially yield better fuel economy and emissions in GCI engines. Five ambient oxygen concentrations varying from 10% to 21% and three different ambient temperature combinations were selected to simulate various ambient environments. A two-color pyrometry was applied to measure flame temperature and soot concentration (i.e., KL factor). In general, HRG flame temperatures range from 1500 to 2400 K under selected conditions. HRG flames have relatively low KL factor for all selected experiment conditions. High KL factors are only observed at the flame periphery where flame temperatures are lower than 1800 K. Accumulated KL factor was calculated to evaluate overall soot amount. Flames at 800 K ambient temperature always have the highest accumulated KL factor. The soot and soot temperature trade-off were also discussed. The desired condition needs to approach a moderate soot temperature with a relative low integrated KL factor level. The conditions of 800 K with 15% O2, 1000 K with 10% O2 and 1000 K with 12% O2 shows better results. The findings can help facilitate the application of high reactivity gasoline fuels in next generation clean combustion engines.