I want to send a password protected email using gmail. Yes I Know I could attach a document, and protect that instead, but I would like to protect the email body, if that is possible?
No, that is not possible with email. Such a function is not specified in the email protocoll. The only possibility would be to encrypt the text in your email body with a tool like PGP or S/MIME. The recipient needs to enter his private password to decrypt the email body. But you need a email client or third party tools to support this (as well as your mail recipient).
If you're interested in encrypting an email with moderate strength but not willing to go through the PGP rigamarole, you could use a Vigenere cipher. It uses a previously agreed-upon word (the key) to apply a rotating Caesar Shift. If I applied the word "protect" to this paragraph, this is the output:
Xw mhy'tx xehxvglivr br ggrimixkgv rb xqcba nwml ohsvftxg liisgkva qlh gsv pxczbri md xc mlthjxv mlg IVG fbkcfpicei, ahj tcnpf nhv o Omixcvfx Gkiwvf. Bx wltj o ivgoxfilpa tvisxh-wide khvf (mwv yxc) vh pgdec c kdkommpz Rrslet Lwztm. Mh B pgdemgw iys pstw "eicmiem" if hamu ipiozvciw, kvbw kl iys hyvijk:
You can use this page to decrypt that or encrypt your own. Note that the Vigenere cipher is not invulnerable to cracking, but it's definitely stronger than rot13, and is a ton simpler than private-public key systems.
SecureGmail chrome extension is pretty fine for this purpose. you can see how you can use it from https://www.youtube.com/watch?v=P-mBcseXV08
with this extension, when you are sending email secure email, you will be asked for a password. and email receiver has to have this plugin too. when receiver gets email, a password input link occurs to let you input a password. after you enter correct password, you can see content of email. otherwise, you see encrypted base64 message.
btw, there is a minor issue. when you start writing encrypted email, email does not behave as secure email. you need to minimise with pressing top-right icon to minimise email (not whole window, just emain input area), then you will see email title becomes red and "Send" button becomes "Send Encrypted".