Call the police first and stay on the line. It documents that you were trying to do the right thing and may be in fear of your life.
Gramps might get there first but he isn't paid to get shot for you despite that he might want to save you.
You get the gun and hunker down. Tell the law you are scared about an ongoing break-in.
You have no idea how it will go down with Gramps. Maybe he gets shot, maybe he shoots the wrong guy. Let the law do that.