There are basically two ways to build a Gmail add-on or extension.
1) Browser extension. The browser extension manipulates the web page directly. The upside of this is that the developer has complete flexibility to inject additional UI functionality almost anywhere on the Gmail interface. This is also a downside, as it creates potential for conflicts if you are running more than one browser extension at a time trying to manipulate the inbox. Probably the biggest problem with browser extensions is they do not work well or at all on mobile, which is where roughly 50% of email is first read. The lack of mobile support is actually quite a large problem as it leads to users checking their email on mobile first and then dealing with the same emails again on their desktop – double the pleasure.
2) Gmail add-on. A native gmail add-on framework was introduced in late 2017 which solves the above problems with browser extensions. Only one add-on can be active at a time, and the UI has been constrained so it works well on both desktop and mobile. The initial support was for additional context while reading messages, and in the fall 2018, add-ons could also have a compose context (i.e. while drafting messages). Some developers may view the UI constraints of the new add-on framework as a problem, but we feel Google is onto something here and we have extended their framework to be usable in Outlook as well.