Typically the point of ballistic gel is to closely mimic human flesh so it will get about as hard a raw piece of meat.Fibre glass is a usually either a fibre or a textile, neither of which have any characteristics similar to ballistic gel. Resin is a substance which will generally harden to somewhere between plastic and glass but always far harder than ballistic gel.To answer if you can use either of the two to replace ballistic gel, "it depends". Depends what use you want from it. Neither fibre glass nor resin are similar to ballistic gel in consistency but if you are looking for another characteristic you might just be in luck. If you want something with a consistency similar to ballistic gel I would suggest something like gelatine, you might have more success with that. Good luck.