Autopilot and GPS problem

Good evening,

I am using the Overland 737ng, which uses the default gps. I have created a flightplan and loaded it into the gps. When I am airbourne and activate the ap, I activate the gps switch, and nav button. I also have dialed in altitude and speeds etc..I remember from using the default planes this would the trick. But now, when I watch the gps screen, I see that thd aircraft roughly follows the magenta line, but not precisely, something like a mile off. What is the problem? I am using the default nav database

I hope someone can help me

Thanks