I did this modification about 8 years ago and its still running strong.
The Hot Glue Gun (hence forth being named HGG) that i owned had two settings "Hot" (would melt hot glue) and "Warm" (wouldn't melt hot glue) but the mean issue I had with this HGG is that if it was plugged in it was on. I found that the "Warm" position was useless, and I wanted to add on/off functionality as well as a power LED. I also wanted it to be done as simply as possible.
Step 1: Materials and Tools Required
1. 0.33 uF 500 VAC rated cap
2. 5mm blue LED (or different color, I used blue)
3. Hot Glue
4. Assorted Heat Shrink
5. Assorted Wire
1. Soldering Iron
2. Second Hot Glue Gun
4. 5mm Drill Bit
5. Heat Gun or Lighter
Step 2: Circuit Bending
Picture 1 is the original circuit it shows the two different heating elements and SPDT switch.
Picture 2 is the modified circuit with an on off switch and LED
Note: The Capacitor being used must be over rated, I personally had a 1KV Mylar capacitor sitting around with the value of 0.33uF or 330nF in a AC setting this capacitor is being used more like a resistor, but for protection I also added a 10K Ohm resistor in series.
Second Note: The lifespan of the LED is being reduced significantly because its being turned off and on at 60 Hz
Step 3: Case Modifications / Assembly
The case modifications on this project are very simple, find a safe spot and drill a 5mm hole.
Assemble the second circuit from circuit bending step, make sure you use heat shrink. Remember this is AC Mains voltage, and can easily cause a fire or kill you. Now hot glue the circuit inside the case of the HGG, and reassemble the HGG.
Step 4: It Works!!!
This is one of those Keep It Stupid Simple projects or KISS projects.
I've had this glue one with these modifications going on for 8 years now and the LED still works so I don't think you'll run into any issues with it.