How do I make 'gibs' using custom props? (Using propper)

I'm making a Minecraft map with some breakable dirt blocks. I have the dirt blocks made already, and I want them to spawn smaller dirt chunks (also made already), but the problem is that I don't know how to make the dirt chunk a 'gib'. Right now, breaking the Dirt block gets you HL2 cement and not my custom model.

I'm using propper for the dirt chunk (it's a working prop). The Block is a brush.

Pictures

http://cloud.steampowered.com/ugc/594715184684130621/88D60FAF01E2AEA9E4DB7EAB02137E8A814890E5/

http://cloud.steampowered.com/ugc/594715184684129747/93833DD1A967217B3691631757D4E968E9DCD701/

Do you know any tutorials on how to go about making custom materials (with the gibs)?