Can GSOC get the Garda Síochána to return my property?

No. If your property is part of a Garda investigation, it will be held until the investigation is complete. Then you must ask the Garda Síochána for it back directly. If you cannot get your property back at that stage, GSOC can look into whether any gardaí were in breach of discipline for not returning it. However, while this could result in disciplinary action against a garda, it is not guaranteed to get you your property back.