In the last few decades a naval gun is typically a complete weapons system "package" and it's installed by whoever builds the ship. But it can be removed/replaced easily enough by any shipyard with suitable heavy lift gear and skilled personnel (naval or civilian).
I don't think any navy has made it's own guns for at least the last couple of hundred years, it's not economically viable. When a steel company like say Krupp of Germany had the experience to make naval and land guns, it's cheaper to contract them to make the guns when you need them rather than maintain a specialized department within the navy to make the guns.
Naval guns are made by various companies and treated like any other weapons system they might make. For example, the OTO-Melara 76mm gun system can be found on various frigates around the world but the same gun was also offered as a turreted package for mobile AAA systems. OTO-Melara supply the gun to whoever is building the ship and maybe that country asks for a few spare systems for attrition or training purposes. It means that they'd be thrown into storage or set up at a naval gunnery school etc. etc.
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