At the moment the gobject-introspection ebuild does not provide multilib support. This is needed for other multilib packages (e.g. libsecret with USE=introspection which is the default).
sjnewbury used to have a multilib gobject-introspection ebuild in his gentoo-gx86-staging repository (see url), but it's been removed lately since it's too old. However, the same principles should apply to the current version.
No, you don't need multilib gobject-introspection. gobject-introspection provides the tooling to generate GIR and typelibs, which is a compile-time native host tool; you won't me calling a 32bit version of it on amd64; the 64bit calls will work for the generation. Also generated typelibs usually also only go to /usr/lib64/girepository-1.0 - it will be picked up from there by python and such use cases, as those won't be multilib either.
