My whole family is-- we'd have to say something if he weren't. DS hasn't been formally diagnosed or anything though, so I guess there's no actual labeling to share.
With DH's family we never use the term gifted, we just tell DH's parents the milestones DS reaches (or back when, sent videos) because grandparents like to know that sort of thing, and everyone else hears from them. I imagine the conversation goes, "How old did you say he was?" Though before we had a child we got a lot of comments from DH's extended family on what smart children we'd have (we have doctorates in mathy fields from prestigious universities and went away for high school), so I imagine they don't need much telling.
I dunno, I guess I don't think the label is so important. What's important is what the kid does with it, or the special needs that come from it. In your case, you don't have to say your child is gifted, just have to mention the child is reading. Let the relatives connect the dots. Or don't say anything at all. "Need to know" is a good way to put it.
