This has nothing to do with the International Criminal Court (ICC). General Gotovina is not charged by that court and the USA has not signed up for that court because of concerns about its univeral jurisdiction, self-definition of crimes, and a lack of oversight leading to "political" prosecutions.
Gotovina has been indicted by the International Criminal Tribunal for the former Yugoslavia (ICTY). The USA supports this tribunal with people and $. Its purpose is narrowly constrained by geography (former Yugo), time period (covers the recent wars), and type of crimes. Further, the UN Security Council (on which the US has a permanent veto) oversees it. US Special Operations teams have arrested Balkans war criminals and turned them over to the ICTY and we still have an element in Bosnia to that purpose.
Finally, go to this link and then click on Indictees by Fighting Faction to see exactly who's been indicted; every faction is included.
Serbs represent the most; it's not hard to figure out why if you know anything about what happened in the Balkans Wars of the 90s.
Thanks for the clarification -- it's hard to keep these International Courts straight, they seem to be growing like mushrooms.