I don't have Gran Marnier for a cake I'm making, will triple sec work as a good alternative? Anything else I should add?

It will work, but Triple Sec is a lot sweeter than Gran Marnier. If you're only using a couple tablespoons, you'll be fine. But if the recipe calls for more, you may want to add a splash of lemon or other acid to offset the added sweetness.

These previous threads on the topic might help: https://food52.com/hotline/15798-a-question-about-a-recipe-chocolate-orange-madeleine-cookies, https://food52.com/hotline/24756-a-question-about-a-recipe-maialino-s-olive-oil-cake, https://food52.com/hotline/4418-a-question-about-a-recipe-clafoutis-aux-framboises