You cant put the gambits in the hotbar. How the gambit works is stringing the correct attack skills in the right gambit order, then hitting the gambit default button. So for instance you have a gambit that shows a shield then spear icon, you would use the shield bash, then use the spear attack icon. These will show up in the gambit default slots, the gambit will change to the appropriate sign, then you can press the button to use the gambit, and your character will that gambit. It can be a bit difficult at first to remember and learn all the different gambits, i had to write them down when I first started. But after a while you will start to learn them. The warden is a fun class, but it is advanced.

Each gambit shows the order of attacks needed using a shield, spear, or fist icon. The number starts at 2, but will go all the way up to 5 depending on your level. Hope this helps.