I've never heard "gain focus". I think focus-in-hook and focus-out-hook
look fine to me. At least, "FocusIn" and "FocusOut" are the terms used
by the X library, and so far Emacs has followed this naming convention.

Yes, it seems that everyone has different names for these events!

When in doubt, we should probably favor GNU/Linux conventions over those

of nonfree platforms.

Brian