As I'm quite the amateur at hoodoo I'll be referring to Moma Sarah's blog and her well-written and informative post about gris-gris.
I will say that from what I've read, gris-gris is considered by many to be the same thing as a mojo bag and I must say I'm not sure if there's a difference. The only thing I can see is that a gris-gris can be made in regards to a specific deity and I have not seen that a mojo is made for that reason.
While you're looking at Moma Sarah's blog, be sure to check out her shop - ConjuredCardea !