What geography her people originated from is hard to determine, due to the fact that natives didn't receive a lot of consideration and were not important in the eyes of the Spanish or the American settlers.
I am that previous poster and MY point was that the Spaniards did not practice institutional slavery as the posted article asserted. There is no question that individual Spaniards and others of that period occasionally captured or "bought" (even from other Indians!) some unfortunate like your great-grandmother and treated them like a slave (and because the victim was a Native American they were more likely to get away with it), but that happens in almost every culture even our American culture (you hear about it even today in the news). It is still rampant in many parts of the world. But there was no institutional slavery in Spanish California. Slavery was against Spanish law (which didn't prevent nefarious individuals from practicing it, just like today) and the missionaries wouldn't permit it when it was in their power to stop it.