At pressure 5.0-5.7 GPa and temperature 1250-1600 ℃, the conditions of synthesis Gem-diamond are reported using FeNiMnCo catalyst by temperature gradient method (TGM), and the P-T phase diagram is given. Temperature field and carbon concentration gradient are simulated based on finite element method (FEM). The results of simulation and experiment show that type-I temperature field is suitable for growing high-quality large sheet-shaped Gem-diamond and small tower-shaped Gem-diamond; however, choosing type-II temperature field, both large sheet-shaped Gem-diamond and large tower-shaped Gem-diamond can be synthesized (type-Ib diamond and boron-doped diamond). On this basis, the growth speed and crystal quality are found to be determined by the carbon concentration gradient, and the rule that high-quality Gem-diamond can be synthesized when temperature field is suitable for the size and shape of diamond growing is given. The pressure and temperature regions for {110} and {113} faces to exist are studied. According to the analysis of Fourier infrared spectroscopy, it is found that the nitrogen concentrations in type-Ib and boron-doped diamonds synthesized by FeNiMnCo catalyst, which are due to the influence of Fe element, are lower than that of normal diamond. The diamond with low nitrogen can be used as an optical material with better transmission characteristics.