You can go with copper or Daconil but I have found both lacking with gray mold. Usually I am only successful in stopping GM if I treat very early and aggressively. I remove any infected leaves by taking the whole leaf stem and then apply a bleach spray. Once the leaves are dry I apply Daconil unless rain is forecast in the next few days; if it is I will use a copper spray as it doesn't seem to wash off as fast. Even if it comes back the aggessive treatment will usually stop it from affecting fruit until much latter in the season. Dry weather and low humidity are the best things to slow the spread of gray mold but then that is true of nearly all tomato diseases.
Bill