Because the land that Disneyland was built on was owned by the Dominguez family, the most well known of which is Ron, who just very recently died (and worked for Disney for almost 40 years).
Former executive VP of Walt Disney Attractions, Ron Dominguez is a “native Disneylander.”
d23.com
If OP was related to Ron Dominguez or was a member of the Dominguez family, his 'grandmother' would have nothing to do with getting a Gold Pass. I'm sure Ron's family has a Gold Pass because he's both a Disney Legend and was DL VP, but as far as I know there was no such agreement with the family before Ron worked for Disney. As the linked article says, he didn't know Walt was using his old house as living quarters and offices, which he would have seen if they had Golden Passes and had been in the park. But he started there as a seasonal worker.
Basically for OPs story to be true, he/she would have to be one of these people: Sheryl Ralston, Steve Dominguez, and Ronald K. Dominguez Jr, four grandchildren Joby Ralston, Tim Ralston, Chase Dominguez and Reid Dominguez. Great-Grandson Brooks. Ron had a brother, Paul, who had two daughters; Jonnie Jean Hofer, who resides in Iowa, and Melanie Harner, who was raised by the Dominguez family in California.