Red gel is a lo-tech solution, a simple sheet of red acetate. All it does is remove all the color information when you look through it so you can see more clearly the tonal values i.e. lighter lights, darker darks. It simplifies the scene to just big shapes so you don't loose track of the overall tonal range. A better choice yet is to use your digital camera and when viewing the picture turn the settings to black and white. And if you are in the studio you can go all the way and use Photoshop/Paint package of choice to 1) load the picture b) desaturate it and c) posterize it to 3-7 shades of grey. I've seen this done by very good and famous painters and it will save you a myriad squints.