1. A card. This is your most basic form of Valentine's well-wishing and gift-giving. From sweet and romantic to satirical and silly, there are Valentine's cards for all types of romances and friendships. While you yourself could google "Gothic Valentine's Card", I would suggest branching out a bit and creating your own. It warms my heart to see personalized cards with inside jokes, snippets of favorite poetry, or doodles. Get thee to a craft store and get creative!
2. A hollow book. While hollow books are fiddly and time consuming to make, they do produce lovely and romantic results. Use their favorite book (you might have to ask a mutual friend) or one that has special meaning to the both of you. Of course, if your love routinely frowns upon the destruction of books, this might not be a fantastic idea, but if not then it's cheap and fun!
3. A poetry reading. This can go either two ways: either you read a poem (that you wrote or a classic love poem) or you take your love to a poetry reading at a club. Because we poets are angsty little creatures, you're more than likely going to stumble onto mostly anti-Valentine's ones or very sad readings. I say, avoid the angst (and the poets there probably won't welcome you, either, if you're going as a couple) and find a funny-themed one. Comedy clubs often have them, though I've seen fliers for ones at Goth clubs as well.
4. A personalized mix-CD. Okay, so we're not known for our love of pink and hearts, but there is something to be said for our love for songs of longing and adoration. Make a mix-CD featuring some of the classic Goth love songs ("Lovesong" by The Cure, "Love you to Death" by Type O Negative, "There is a Light that Never goes Out" by The Smiths, "Friday, I'm in Love" again by The Cure, and "Hello, I love you!" by Siouxsie and the Banshees come to mind.) and present it to your dearest. Bonus points if you add a track of you singing on it.
5. A cute, Goth-y stuffed animal. Disclaimer: not all Goths like and appreciate stuffed animals or plushies, but if your sweetie is fond then you have a lot of options. From plush zombies to bat-like ugly dolls, you have a lot of options.
6. Flowers. This is such a cliche I can't believe I'm writing it, but it holds true for many Goths that black flowers (especially, though not limited to, roses) are a cute gift. However, and this is a pet peeve I find with a lot of people that give flowers, don't present it wrapped in plastic! Take some time to wrap it with tissue paper, lace, cloth, etc. If your beloved is oh so spooky, they might appreciate some effort into making the flowers oh so spooky, such as gluing a plastic spider to it or staining white petals red with "blood." Make it as special as your Valentine is.
7. Anatomically heart-shaped anything. Many people, including Goths, have a kind of morbid fascination with the actual shape of the human heart and companies have started taking notice. There are anatomical heart necklaces, candies, Valentine's cards, and so much more. If a pun is involved, you get double points from me.
8. Molded chocolates. You can find heart-shaped chocolates just about anywhere, but if they come in bat or skull shapes then all the better! There are many tutorials online for molding chocolates, so get googling! I particularly love these ones, with walnut brains! (Hint: make sure your Valentine actually likes chocolate and is not allergic to it or any other ingredients you put in.)
Of course, you should never feel pressured to get your beloved something material for this holiday. If they're worth your time, they should be satisfied with your presence and well wishes. I hope you all found someone like this and that you had a very happy day!