Mass is of a single particle is extremely small (proton has a mass of 1.6e-27 kg), speed is very high and already very close to the speed of light at all but the lowest energies, so gravity "assists" are not really anything with which you can gain energy.

These are charged particles (protons and fully ionized heavier nuclei), so electromagnetism is a much more efficient way to reach high energies.

All currently proposed mechanisms for cosmic ray acceleration involve turbulent plasmas and shock fronts that reflect particles magnetically, giving them a small bump in energy each time [1] or rotating magnetic fields [2] in sufficiently extreme environments.

[1] https://en.wikipedia.org/wiki/Fermi_acceleration

[2] https://en.wikipedia.org/wiki/Centrifugal_acceleration_(astr...

Gravity just doesn't play a direct role for accelerating charged particles, it's much too weak compared to the electromagnetic force for a charged particle.