I highly doubt you will find anything pre-built that does exactly what you want. Also, I suspect it's not possible to interact with Gmail's Canned Responses mentioned on Sixty4Bit answer.
The solution is to write your own plugin with a library like Inbox SDK:
If you're going to write a web or mobile app then you need the official Gmail API.
For the sake of completeness, I'll mention Gmail.js but it's not able to send emails.
While researching for this answer, I came across a very interesting discussion. On that thread, both authors commented:
Creator of gmail.js here - thanks Nathan! I started working on this almost 3 years ago for a personal project. At that time there weren't many libraries available to work on top of gmail so I decided to compile all the helper functions into a standalone library hoping that after it goes on github, others might contribute.
The whole thing is intentionally designed to be a single file where majority of the functions are standalone so if something goes wrong, anyone can easily understand how it works! That being said, a lot of the focus of this library is to abstract network events so developers making extensions on top of gmail can fire off events when a gmail user gets a new email, sends email, deletes etc. Those things rely very little on the DOM.
Founder of Streak here (we make the InboxSDK) - this is one area where we take a slightly different approach than Gmail.js.
We host our SDK and you remotely load it at runtime. The benefit is that we detect breakages due to gmail and automatically update the SDK. Your users just need to refresh gmail to load your extension and the latest SDK which is compatible with any changes Gmail makes.
Gmail.js lets you avoid doing the DOM hacking yourself but is still fairly low level. The InboxSDK was targeted for people who are building apps inside of Gmail. We assume multiple extensions to be running and make sure they play nice, we handle auto updates to handle changes in Gmail and we try to guide developers to using standard Gmail UI styles.