Correct, Able doesn’t come with a stat system but is written to hook into any stat system rather painlessly. Able simply asks you for the final value to apply as damage, or cooldown reduction, etc - and you can do whatever math your stats system requires.
You can use Gameplay Ability System’s Attribute sets, but if you want to modify those attributes from within Able, you’d need to craft a special task that allows you to apply GAS effects to those stats.
You can also find a stat system on the marketplace you like and that should work just as well.
In my own projects, I just have a very simple stat system since my games don’t require anything crazy.