I am more obsessed with the outside of the home than the inside and I also love gothic atmosphere. There are a lot of wonderful ways to add creepiness, mystery, magic and frights to a garden.
You might want to begin with wrought iron. Nothing says cemetery, Victorian Era, and Gothic better than wrought iron fencing, gates, furnishings, and details.
Think in terms of hidden spaces and overgrowth, shadows and structures. Cryptic-that's critical for spookiness. It's what you can't see, what you have to meander to find, what you have to poke around in to discover that keeps the mind uneasy.
Architectural details can give a very spooky feel, especially if they are worn, broken, and look like you stole them from an ancient church.
The last thing you want to do is something neat and modern. Look at how the stone is broken as if it were ancient and yet left there in place as if no one had tended to it in centuries. Let vines grow up. Use brick for the ground that is beaten down on the edges and old looking. Let moss take over. Allow paths get to hidden and limbs to block off quiet areas. Add bird baths and gazing balls, fairy houses and even scarecrows.
Broken glass windows, aged shutters, finds from abandoned sites all help to make creepy factor, including a tiny little shed made from old boards and found things that makes one wonder at its origins and age -
Sometimes, adding magic to a garden is enough to make it mysterious. Tree faces, sculpture, boxwood mazes, arbors, vines and even labyrinths can give character and beauty, as well as unsettling creepy qualities.
Gnomes are freaky garden inhabitants, but what if you put in a lot of them in a grouping?
Lighting is critical for a creepy garden, whether it's a fire pit, chains of tiny Christmas lights, Wrought iron lanterns, or solar lights shaped like old-fashioned gas lamps.
Think in terms of sound, as well. Windchimes can add a magical life to a garden that makes it seem like a sentient, moving, changing being. I was filming an abandoned site one time and a sheet of rusted metal hung from an eave by a single bolt and the wind made it swing and creak. The sound was chilling. Very nice touch to hide in a garden somewhere.
Bamboo windchimes have a mystical spiritual sound to them, but the large metal long tubes like the ones above create a very churchyard sound that is compelling.
Other elements to consider adding are angel statues and other statues that look cemetery in origin, gargoyles, sculpted bushes, Green Man, as well as stained glass.
A catalog that might inspire is Design Toscano.
Here are some things I employed in my garden in the past when I was married and lived in a house. -
Giant hand-made windchime
Mock cemetery in the corner with lanterns.
I made some styrofoam "orbs" by fishing string in the garden as a nod to my ghost hunting -- ironically, a poser orb showed up in the pic. Hee hee
Deciding a theme is a great way to go. Magic? How about Green Man, fairies, gnomes and symbols? Medieval Gothic? Windows, doors, stained glass, worn stone, and gargoyles. Horror? How about a chicken wire ghost or a few ghost figures for the garden?
A garden zombie crawling out of the ground.
Let the features of the land speak to you and don't be afraid to remember to let plants go wild, think vertical and horizontal, lighting, sound, and cryptic features. Gardening should reflect the homeowner and paranormal geeks seriously need a nod to their varied obsessions.