Could I leave out the Grand Marnier or substitute something else?

Actually, all the alcohol will not burn off - so if zero alcohol is the objective, leave it out.

I recently asked a similar question - I've been making clafoutis for years, always with some kind of liqueur (kirsch, grand marnier, etc.), but was cooking for someone who really could not have any alcohol. Period. So, I used vanilla bean as the flavoring in a pear clafoutis - and it was delicious without the liqueur.

You could also use some grated orange peel to give it some orange oomph.

If you are worried about the alcohol, try a little Seville Orange Marmalade to get a similar taste, but of course, not the same thing

You may substitute for a raspeberry liquor or other fruity liquor. Ive never done it without and it really enhances the flavors. If you're worried about the alcohol content it does burn off while in the oven! Hope it helps!