make sure the gar has some size is my only advice.. I seem to recall gr8(?) having a ray tasting a gar.. gars are also pretty skittish in general.. and don't try it with a gator..
My Florida Gar gets along perfect with my rays. He is probably my most laid back fish though...He's super cool! In my experiences, as long as all are properly fed and there is ample room in the tank, you should be fine.