I'd like to receive all my emails with content-type
text/plain by default while using gmail (no clients), by default gmail uses content-type
When you receive email in gmail, gmail honors the "text/html" body part if one is present. I do not think there is any way to disable this behavior if you are using gmail's web interface.
However, for mail that is "sent from gmail", this is easily do-able:
I checked my gmail, and it is sending mail with:
My Gmail "Settings --> General" has this enabled:
And when I compose a message in gmail, I make sure I am using "plain text" for my email.