What kind of sauce would you serve with a turkey breast cooked on the grill (since there will be no drippings for gravy)?

Thinking outside the lines a bit you could do a Mexican style pipian, which is a pumpkin based mole type sauce. Mexico is probably the only country that enjoys turkey as much as our country does.

More on the fruity sauce suggestions above:
Pear, Brandy and Walnut Cranberry Sauce
https://food52.com/recipes/1467-pear-brandy-and-walnut-cranberry-sauce
This is more like a traditional cranberry jelly (very delicious)
https://food52.com/recipes/19725-canal-house-s-cranberry-port-gelee
There are a few recipes for mostardas on the site. You could use frozen cherries for this - just make the cherry "sauce" part of the recipe.
https://food52.com/recipes/22902-seared-pork-tenderloin-with-skillet-cherry-mostarda-mint-rice-salad

Any chutney, we like pineapple or rhubarb (hard to find rhubarb this time of year). Make your own or in a pinch, purchase one ready made.

Some type of fruity BBQ sauce would be nice.

Just read Alice Waters' Parsley and Anchovy Sauce article. That sauce would be amazing with the grilled turkey breast, and completely different from the traditional turkey and gravy you are also serving.

You could buy some turkey parts ahead of time, roast them and use those pan drippings with turkey stock you've also made/purchased ahead of time. Gravy all done, no last minute prep.

I originally thought some fruit based sauce would complement the turkey. Your olive oil, garlic & rosemary suggest an Italian note, and mustard makes me think of mustarda di cremona - a northern Italian condiment with wine, fruits and mustard. Another sweet/savory balance for the turkey is any chutney, but especially nice this time of year is one made with cranberries.

If you have the giblets, use those as the base for a pan sauce. Give them a hard sear on all sides and use that fond for the flavor of a "faux" gravy.
Otherwise, use pancetta or bacon.