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.
What about using the Gmail API into an app that gets overlayed into Gmail via chrome extension?
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.