Actually if you don't want to buy a bottle for a single teaspoon, you could go to your local liquor store or Beverages and More (aka BevMo) and get an "airline" bottle of Gran Marnier. I do this frequently for recipes that call for just a little. And really, there is no good substitute for Gran Marnier. :)
Grand Marnier is an orange flavored liquer. There are many substitutes out there and one of them is called Cointreau, which may cost as much as Grand Marnier. Any orange flavored liquer should do the trick. Ask your local liquor store for some less expensive alternatives.