There used to be a lab for fixed-with font for text/plain in Gmail, but it had been removed. This is really a shame.
You can complain here: Default font for plain text message
Meanwhile, you can also install a Chrome extension: Fixed width text in Gmail™. You can then change the particular fixed-with font in Chrome preference (direct link for Chrome users), but beware: it will apply to every website, not Gmail only.
Coupled with extension synchronization, this is a passable solution. It will still not work for mobile access. It is not worth a native lab, but that’s all we have…
In order to optimize the chance of being reading, I also opened an issue at Gmail’s help forum. Maybe Sara Goetz will read me.