I've sent feedback as well.  In case it's useful, here's what I sent to Google:

--------

I sometimes compose text for a message in another app, then paste that text into the body of my message in Gmail.
Often, Gmail loses the line breaks in my text.

The developer of Textastic (app for iPad) has identified the problem in Gmail. They say:

"
The Gmail app's mail composer doesn't handle plain text correctly and doesn't insert
tags when it encounters new line characters in plain text.

This is a bug in the Gmail app.

I can reproduce it when copying text from these other apps and pasting it into a new email in Gmail as well:

iA Writer
1Writer
GoCoEdit
Runestone
"

Since we know that Google is reading everything we type, maybe we can consider these bugs in their apps to have been reported already. 8-)

I noticed tonight that the same behaviour occurs when pasting text copied from Textastic into Google Keep.