Open up a blank text document, do some typing & see if the same thing happens. Perhaps one of the keys on your keyboard is wearing out (or dirty more likely) & is sticking. Another possibility is the mouse being dirty or wearing out & doing actions you are not aware of.

The so-called Mac Mighty mouse tend to have a short life from what I've read & experienced for example. My mac keyboard I use a 3rd party keyboard cover for which is very good (I simply wash it under water occasionally) & does not affect typing whatsoever.

You could try typing into a text document, when finished, copy-paste the text to the gmail compose mail's window.