When she is with you, get her in the habit of going to the bathroom at regular times, no matter what, no excuses. Use a timer or alarm, say for every 2 hours. At a bit younger, the doctor said that bathroom breaks every 2 hours were a minimum requirement for kids. This was a school issue with my son due to an idiot teacher, but the info has stuck in my brain, 2 hours for kids and at least every 3-4 for adults.
Why set a timer and establish this habit? You are training her bladder and her brain. It will hopefully help. One thing people with adhd do is hyperfocus on things they are really interested in. They ignore everything else. For all intents and purposes, nothing else exists. When they focus so intently, they are not paying attention to their bodies or anything else except what they are doing, be it a game, movie, etc.... Their body may fidget but they don't know it. Their brain is in the game or whatever. My oldest focuses that way or else he does not focus.
This is sort of typical of adhd.
She may not be adhd, this isn't the way to tell. Many kids do the same thing because they don't want to stop the game or whatever. Often if they stop, Mom/Dad/Teacher/Adult-in-Charge won't let them come back to it. So they don't want to stop in case they cannot come back.
Use the timer, but don't make it a punishment or the end of her fun thing. If she has to stop something, make that thing stop at a time different from the timer. Or use a free alarm clock program on the computer that plays Christmas Bells for the toilet and Jungle Roars for stopping the game. Just something different and a different time to stop each thing. Let her come back to her game for a few minutes, PITA as that can be to stop again, after she goes to the bathroom.
Holding it until you dribble can cause health problems and infections. I had a neighbor growing up who did this and he ended up in the hospital. He just kept holding it and holding it, and his bladder got irritated and then infected to the point he almost died. I am not saying your granddau will, just that this habit should be started early. My neighbor's parents were read the riot act for not getting him in to see a urologist earlier. He did the wait until he dribbled thing for years - he was a teen and was known to do it so long that he had some ugly nicknames among the older kids.
Push your daughter to get her into a urologist to rule out a structural or physical cause before they look for a psychological cause. Psychological causes should always be the last one you look for, not the first. Establishing regular toileting habits can go a long way toward helping though.