The gambeson is a padded jacket, stuffed with rags or fiber. This one is an early 14th Century style, with a high collar and long sleeves. There are leather cleats over all the joints for lacing on armor.
Later gambesons would become thinner and more fitted, serving mostly as a base for tying on growing sections of plate armor.
In this drawing from from the Manesse manuscript, which is dated around 1330, you can see the long cuffs and high collar of the gambeson.
The 1322 regulations of the Armourer's Company of London listed in ffoulkes suggest that the terms "aketon" (from Arabic al-qutun - cotton) and "gambeson" were interchangable by this time: "That is to saie that an Akton and Gambezon covered with sendall or of cloth of Silke be stuffed with new clothe of cotten and of cadar and of oldn sendal and not otherwise. And that ye wyite acketones be stufed of olde lynnen and of cottone and of new clothe wth in and wth out." Other later citations in ffoulkes mention coverings of buckram, a material also used on wagon covers, so some sort of canvas would be expected.
This picture of a gambeson is taken from the Maciejowski Bible of 1250.
I made the gambeson from Period Patterns #101, "Medieval Military Garments."
There are two schools of thought on padding. One is to use layers of cloth and batting and stitch them together. This is much quicker, but limits your padding to the thickness you can stitch through. The other is to sew the cloth cover into channels and stuff them with fiber or rags. This is harder, and takes longer, but gives you that ever-popular Michelin Man look. It can be cheaper, too, if your stuffing is recycled. BUT, whatever you use, particularly if it's loose fiber, beware of settling.
I used rags, which is period. Specifically, I used old towels cut into strips and rolled, then pulled through long channels. (The terry cloth gave good loft for the weight, but the result was still pretty massive. A large pile of towels disappeared into the gambeson and cuisses.)
It was hard to pull the long strips for the torso and sleeves without the hook coming loose.
The friction was high, the surface area was huge, and the tip that you had to hold on to was tiny. I had to use two strong clamps at a time, of the kind used for bib overalls, just to keep a grip on the padding as I pulled.
But for all the fight that the rags gave me going in, the steady pull of gravity was able to slide them back down again. In the few days that the coat hung on the back of my sewing chair waiting for its buttons, the padding slumped down as much as 4" from either side of the shoulder seam. There was no padding left on the shoulders where it was most needed!
I had to remove the arms and rip the shoulder seams, pull the padding back up, stitch it in place, and then re-assemble everything. What a pain! Be warned.
By the way, speaking of shoulder seams: if you plan to attach your armor to the gambeson with arming points, make sure those shoulder seams are strong enough to carry the weight of the couters and vambraces and such. Blows to your arm or even your shield will translate into tugs on that shoulder seam, and can tear the arm right off your gambeson.
Also, you don't want to stuff a whole bolt of cloth and lay out your pattern on it. You'll want to stuff only the pieces you will use. But, that raises the problem of shrinkage.
Stuff a sample section and see how much wider the flat fabric is than the stuffed. Then, when you lay out your pattern pieces, make them wider by the same percentage. You'll have to measure key points and sketch in the outlines between them. Later, when you stuff the pieces and they look like the original pattern, you'll feel like a math wizard.