1. GMail-mode

Gmail mode sets up a number of fallthrough keys to allow you to use Gmail's keyboard shortcuts. It also provides the command gmail-label-goto on C-c C-g to jump to a specific label. To use it, put the following in your rc:

require("gmail");

2. GMailTeX

2.1. automatic

With gmail-mode enabled, the following code enables gmailtex:

var gmailtexjs = "URL-TO-GMAILTEX.JS-HERE";

function gmailtex_enable (buffer) {
    var doc = buffer.document
        .getElementById("canvas_frame")
        .contentDocument;
    if (doc && (! doc.querySelector("script[src='"+gmailtexjs+"']"))) {
        var s = doc.createElement("script");
        s.setAttribute("type", "text/javascript");
        s.setAttribute("src", gmailtexjs);
        doc.getElementsByTagName('head')[0].appendChild(s);
    }
}
add_hook('gmail_mode_enable_hook', gmailtex_enable);