The Gmail Bookmarklet will add a unique string to the email signature to prevent Gmail from trimming the content. The hex color code for the appended text is #444 so it will not be prominently displayed in Gmail.

/* Gmail Trim Bookmarklet */

/* Are the ellipsis shown in the Gmail message */
var showtrim = document.querySelector('.aH1');
if (showtrim) {
  /* If yes, click to expand the trimmed content */
  showtrim.click();
}

/* Does your email message have a signature */
var gmail = document.querySelector('div.gmail_signature');

/* Get the unique message ID assigned by Gmail */
var id = document.querySelector("input[name='composeid']");

if (gmail && id) {
  /* Append the current Date and ID to the signature */
  gmail.innerHTML +=
    "<br><small style='color:#444'>Message #" + id.value + ' sent on ' + new Date().toString() + '</small>';
}

/* Written by Amit Agarwal amit@labnol.org */
/* For updates, see http://www.labnol.org  */