I tend to use phrases like "better fit" and "asynchronous development"... if it comes up. I tend to avoid the topic unless I know that I'm among other parents of GT kids.

If they ask what school/grade your child will be in next year, just be matter of fact and say he's going to be in XYZ school (no need to mention gifted) or nth grade. Then change the topic to their kids by asking about them.