Both. The product information lists post-exercise use, massage after gardening or desk work, and times when the body feels tense from prolonged sitting as appropriate occasions. It also notes that such gels are often used before or after activity, so either time can work depending on what you want from the massage.
After exercise: take a shower first so the skin is clean, then massage the muscles you trained. The gel's spreading texture makes it easy to cover legs, shoulders or back. The cold-warm contrast and the massage itself may help the area feel less stiff and more mobile.
Before activity: a short massage with the gel can be part of preparing the skin and muscles for movement, but it is not a substitute for a proper warm-up. It works as a cosmetic massage aid, not as a painkiller or performance product. Start with a small amount and stop if it feels uncomfortable.
For desk work or travel: massage the neck, shoulders or calves during a break. Because the formula is non-greasy, you can easily dress afterward, but wash your hands if you are going to touch your face or eyes. No matter when you use it, avoid applying to injured, swollen or irritated areas; if pain is persistent, unexplained or severe, consult a specialist rather than relying on a massage gel.