Movie is not about psychological issues
If it wasn't for that Simon then it could been something else. But having simon on both mary and gordon basically tells that place is haunted supernatural force that went to new victim. Gordon didn't go crazy because of mental issues but rather paranormal one instead.
If mary and gordon were somehow connected like relatives then it would be way more intresting because it could have been genes releated. Well it would still be bit too far off but could be possible who knows.
It also went really slow too, there is too much things in this movie could cut off and maybe something more releated added. I think this movie could have had more potential with story too. I'd give it 7/10