Porous layers on (100)-oriented n-type liquid encapsulated Czochralski grown GaP crystals were fabricated by electrochemical etching in a H₂SO₄ aqueous solution and analyzed by scanning electron microscopy. ¹²C⁺ ions were introduced at room temperature by 3 MeV energy implantation into porous and bulk samples at two ion doses of 10¹⁴ and 10¹⁵ cm^-2. The prepared samples were annealed in the temperature range between 200 and 600 °C applying rapid thermal annealing (RTA) technique. A comparative micro-Raman study was carried out on the porous and bulk substances. Porosity was found to lead to the violation of the selection rules and to remarkable changes in the optical properties. Additionally, Fröhlich-type modes were observed in the Raman spectra of the porous layers. High energy implantation produces a thin high damaged layer, buried at the depth of the mean projected range. Implantation does not result in a drastic damage of the samples and they undergo a fast recovery after RTA. After this treatment a semi-insulating GaP layer is created, which is thermally stable up to 600 °C.