User-defned materials are supported for SPG in the same manner as for solid elements. *MAT_ADD_EROSION is also supported for SPG. However, please use LS-DYNA R14.
Note that the function of *MAT_ADD_EROSION is different with that of FEM. This keyword will delete an element in FEM when the criterion is fulfilled. In SPG, the connection of two particles (the bond) will be cut when the criterion is fulfilled, that means you will not see particle erosion.
I hope this helps.