Okay, so, for now I'm going to propose 3 tiers of Gambits. Level 1, level 5, and level 11. If we want to add epic gambits later on, cool, but for now this works fine.
In addition, I think one of the primary Gambits should just be Improved Martial's Gambit. Simply, you spend 1 mettle and target 2 allies with Martial's Gambit. You can either then just make your attacks, or you can really burn through mettle and also do an Attack Gambit like guided strike. There is a really solid basic action economy and resource strategy in choosing your attack(s), your bonus action, and whether to boost the attack action, bonus action, or both, with your mettle.
Let's decrease the number of gambits per tier, and figure out how we want the gambits to scale, instead.
So, Guided Strike. What should it do at level 5? Should it simply do more for the same cost (the ally attack gains a damage bonus equal to your Intelligence mod at level 5), or should it have a "spend 2 mettle instead of 1 and allow a second ally to attack" thing?
To compare to our chassis class, the monk gets Stunning Strike instead of these upgrades. The problem with SS is that it becomes the most important use of ki, and replaces anything else you might want to do, in terms of optimization, unless your tradition gives a really solid boost to Flurry of Blows, or gives you spells to spend ki on.
Thus, the idea of adding the same sort of power boost, but adding it in a way that doesn't make the class compete with itself as much, and keeps the class from becoming "always spend mettle on X ability".
The other big question is, should we present these as a list of things to choose from to learn, or simply, "at this level your gambits improve, and you learn these 3 new gambits"?