I'd use the whole black peppercorns rather than ground white pepper. Many gravalax recipes use black
The first time I make a recipe, I make sure I have everything the way the author calls for... Otherwise there is no way to know if you've gotten it right. After the first foray into a recipe, all bets are off. If this is your first time making gravlax, I'd go out and get some white peppercorns. Frankly their flavor profile is completely different from black. and there are more than one kind of black pepper.
Lots of gravlax recipes have black pepper, and lots have ground pepper. Some have no pepper at all. I'd say you should go with whatever you'd like.
I'd say if you'd like to have a bottle of white peppercorns around, go for it. Might come in handy. If not, I can tell you that I've made gravlax many times with black ones. Different flavor, but still very good.
Another vote for staying away from the ground white pepper. I too hAV made gravlax with black peppercorns
White pepper is very different in flavor so play with a small batch if you can to test