It wouldn't have a gaping hole in it. The appearance of a planet after a major collision is very different. Basically, if the collision was energetic enough to "gouge a hole", then the entire planet is simply melted instead. And on that scale planets are not rigid, anyway -- if you put a big hole into one then the rock will flow until a relatively spherical shape is restored, like anonymousgamer says.
Anyway, collisions on a planetary scale are probably not something you're going to see in SE for a very long time. Implementing them is an extremely difficult computational problem.