Complex sentence: Not all of that energy can be utilized by any realistic propulsion technology because, while electron-positron reactions result in gamma ray photons, in reactions between protons and antiprotons, their energy is converted largely into relativistic neutral and charged pions.
Split into simpler sentences: Not all of that energy can be utilized by any realistic propulsion technology because of the nature of the annihilation products. In reactions between protons and antiprotons, their energy is converted largely into relativistic neutral and charged pions.