One possible solution is to look for a font where these letters aren't wider than the basic English letters (or to phrase it the other way around: the English letters are also as wide as the Amharic ones).
I'm not familiar with Amharic fonts, nor with
fontconfig's details, but chances are that the English and Amharic fonts are taken from two different font files, and some fontconfig rule merges them to appear as one towards applications. You might study fontconfig and somehow tweak its configs to merge the Amharic fonts with slightly larger English ones. I'm not aware of the details, nor am I absolutely certain that this is doable, but I believe it is.
A much simpler workaround is to go to gnome-terminal's preferences, and increase the
cell spacing. This option newly appeared in the version shipped by Ubuntu 18.04.