Isotactic polypropylene (PP) under atmospheric pressure usually crystallizes in the monoclinic alpha form, and also in the trigonal beta phase, e.g., using appropriate nucleating agents. Crystallization of the orthorhombic gamma form is facilitated by high pressure (P), although it also requires high temperature. This phase has a unique structure with non-parallel chain alignment. Recently, we demonstrated that alpha-nucleants can nucleate the gamma form under high P at 200 °C. During processing crystallization is non-isothermal, hence the present study focuses on nucleation of the gamma form in PP under high P during cooling. The study utilized PP 3250MR1, Arkema. Three commercial alpha-nucleants, (Hyperform HPN-20E, Millad 3988i, PTFE submicron particles), and a known beta-nucleant, calcium pimelate synthesized by us, were used. Neat PP and PP with 0.2 wt% of nucleant were crystallized in a high-pressure cell, under P ranging from 100 to 300 MPa, during cooling at approx. 8 °C/min, and then analysed by different methods to examine crystallinity, contents of crystallographic forms and spherulitic structure. Crystallization temperature (Tc) of all the materials increased with increasing P. PP with HPN-20E and PTFE exhibited the highest Tc; under 300 MPa Tc of PP with HPN-20E exceeded that of neat PP by 14 °C. Both HPN-20E and PTFE increased also the gamma content and reduced the spherulite size. Calcium pimelate had no such effect; it increased only the beta content. The effect of Millad depended on P. Under 100 MPa it elevated Tc and the gamma content, and strongly reduced the spherulite size, but the effect vanished with increasing P. The results show the possibility to nucleate the gamma form of PP under high P during cooling, hence to increase its content and Tc, and to reduce the spherulite size.