Gambit is a mutant:

Molecular Acceleration: Remy has the power to take the potential energy stored in an object and convert it to kinetic energy thus “charging” that item with explosive results. He prefers to charge smaller objects, such as his ever-present playing cards, as the time required to charge them is greatly reduced and they are much easier for him to throw. The only real limitation to this ability is the time required to charge the object. The larger it is the more time it takes. He can charge a rock as big as a small house in the matter of seconds. Most charging takes place through direct skin contact.

The power of his explosions is dependent on the mass of the object he is charging, for example, a charged playing card explodes with the force of a grenade. Gambit can also use his powers to accelerate an object's kinetic energy instead of converting its potential energy, for example, he can charge his bo staff with enough power to level a house. His ability also allowed him to affect his own capability to heal but this is rarely seen, he's also used his power to accelerate organic material on a few occasions either causing them to explode or enhancing their own physical capabilities. These powers have been augmented once by Sage to enable him to accelerate his own regeneration process and better control his kinetic shifting and again byFaiza when he was shot through the head by a crime lord and spat a bullet back at him that eviscerated the later.