Although it launched as paid DLC in 2019, Man the Guns has long been integrated free into the base game. It still deserves its own file because the ship designer it introduced is the foundation of all naval customization today.
Nations and Content
The United States and the United Kingdom got their first serious focus trees with this DLC, plus content for Mexico and the Netherlands.
What Changes
The big addition is the ship designer: instead of picking from a handful of fixed models, you assemble each ship class piece by piece (hull, armament, armor, systems). It also introduced governments in exile for occupied nations, naval mines, and naval treaties that limit your fleet's tonnage if you sign them.
Is It Still Worth It?
The question doesn't fully apply anymore — being integrated free into the base game, there's nothing to buy. But it's worth knowing that the ship designer, governments in exile and naval treaties come from here, to understand where those mechanics originated the first time you run into them.