I think it depends on the purpose of the bag. Gris Gris bags are used in both faiths in a similar fashion and for similar purposes...typically consisting of a small pouch containing items specific to a spell, sealed, and either worn or carried. These bags are not meant to be opened for any reason in either faith.
I have several small bags that I keep herbs and stones in, but they are not Gris Gris. These are meant to be opened when I need to use the items within.
Many Christians will tell you that Voodoo is a form of Witchcraft, but that's a misconception on their part. Anything involving magick is "witchy" to them.
While many westerners view Voodoo in a negative sense, Voodoo/Vodun is a highly respected religion in other parts of the world. In modern times it has been mixed with Christianity and many Christians in Latin America and Africa practice Voodoo as part of their beliefs and rituals. Santeria is a good example of this mixing.
The main difference between Voodoo and Witchcraft, I think, is the basis of the beliefs. While Witchcraft centers around nature, Voodoo focuses more on ancestor worship and spirits of powerful ancients. Both beliefs fall into the category of Paganism, but the similarity stops there.