It is, but it's an inferior proposition for our business (and for users) without access to the Gmail store. In fact we used to have our functionality as a Chrome extension but Google aggressively marketed the native approach when they published the "New Gmail". And we went for it.
It seems like add-ons are still mostly Ok if they use the narrowly defined scopes, instead of say "gmail.readonly" which is a blanket permission. Minor heart attack, but we might be ok.