By looking at CS mod code, yeah, you're right; there is no associated classname to the CGib class. An easy way to hook Gib spawn would be hooking SetModel (checking the properly model)
Then with the gib entid, you can hook with RegisterHamFromEntity forwards you want. But at the first hook, you wouldn't be able to hook Spawn pre (because you'll be registering forwards just some steps after Spawn Pre).