Using gel during a treatment is in itself neither good nor bad. Using gel under a misguidance of what it actually does, inevitably leads to a negative effect in your results. The greatest benefit in the use of gel during a laser or light treatment is that it enables the applicator to glide more smoothly over the skin.
Using a gel as coolant for epidermal protection is not the most efficient epidermal protection you can offer your client. Skin cooling during a photothermal treatment is required to remove the heat from the epidermis. This is best accomplished by active cooling. It can be in the form of a water or air-based skin cooler, or even an ice pack. Using this type of skin cooling allows for the heat to be extracted and removed from the epidermis. An alternative is also cryogenic cooling. With this method of cooling, the skin is cooled right before and after it is heated.
Using gel as skin cooler does not allow for the heat to be removed. It creates a layer on top of the skin where the heat can conduct to. But then it is locked into the gel layer. As the treatment continues more and more heat is locked into the gel layer, causing an overall increase of epidermal temperature. This process can be reversed if you combine gel with a form of active skin cooling.
Another reason why therapist believe in the necessity of gel, is for conductance. This is an idea derive from technologies like ultrasound and micro-current, where conduction is of the utmost importance. With light therapy, absorption of the light is of the utmost importance. Any barrier between the applicator and the skin (or target) can have a negative influence on absorption.
The gel layer reflects and refracts the light. Depending on the wavelength and gel used, it can also absorb some energy.
To conclude, if you are using gel it is best to use the thinnest possible layer of gel to ensure minimal interference with your treatment results. If you use active skin cooling in combination with gel, it is important to remember that you still need to allow time for the cooling to be effective. If you have an applicator with build-in skin cooling that you glide over the skin on gel, you need to allow more time for a given area to be cooled sufficiently.