Honestly, the breeder not knowing what dad was is a big red flag to me. Especially if this was being advertised as a garg from the start. Sure, sell it as a "Sterling Pastel + ??" but to breed pewter x ?? and just say Garg is really not a good thing to do.
If the breeder doesn't know what the genes are...the only way for you to know for sure will be to raise this little one and test breed it out.
Nikkubus and Albert added some good input on the look of the baby, so hopefully it does prove out as a lace black back, green pastel or HRA in the end. They've both have good eyes for this sort of thing.