From this time gambeson came to mean a padded jacket used to protect the torso (and generally arms as well) from bruising blows. Historically the gambeson was a foundational garment worn over armour or on its own and would have been worn by those who could not afford better.
Historical gambesons could be made of soft material (such as used linen or tow) stuffed into a couple of layers of tougher fabric such as linen. It might also be made of many layers of material. One source indicates that 30 layers of linen make a sufficient garment.
Gambesons in the SCA
In the SCA, many fighters do not wear gambesons or heavy armour, taking advantage of the SCA's rules to maximize their speed and mobility. When a gambeson is worn, it is generally worn under a heavy fighter's rigid armour, and for fencers it is generally worn just by itself with minimal added protection to groin and joints.
Ideally, a gambeson is be made of tough, heavy fabric on the outside, and soft, comfortable fabric on the inside, with a layer or two of quilting between them for padding.