"End-to-end" implies that Gmail won't be able to read your emails. That means that this software and Gmail, one of Google's largest products, are going to be competing. One of them needs to adapt or die: if this software isn't backdoored or vulnerable right now, it will either be shuttered, backdoored or made vulnerable in the future. (Certainly Gmail is of tangible, financial good to them: it's more likely for them to favor it over End-to-End, which is a non-profit and humanitarian effort.)