In Modern Warfare 2019 and the past two Call of Duty titles, the Gunsmith is meant as a way to add attachments with specific stats. Each attachment will usually have a pro and a con o that weapon builds become important and refined. To date, the Modern Warfare version is still one of the most customizable.
Modern Warfare and Warzone 2 - Gunsmith platform feature

One of the biggest changes to the Gunsmith in Modern Warfare 2 will be the additions to platforms. To an extent, the idea of platforms already existed in Modern Warfare 2019 and was removed from subsequent games. In essence, using certain attachments would change a gun entirely.
For example, changing the barrel, the ammunition, and the stock of the AK-47 could create two different weapons. One could be an AK-74u or the other an RPK light machine gun. In Modern Warfare 2, this will be consolidated into a platform system.
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An AK-47 would be an example of a platform in the upcoming games. Using the base platform will allow for more weapon trees that change the purpose of the platform. Equipping the sub-platform will then allow for unique attachments that weren't available before.
There is no confirmation on the number of weapons that will appear as platforms in Modern Warfare 2 and Warzone 2. Whether the platforms affect the overall weapon count remains to be seen, but there is sure to be more customization.
Modern Warfare 2 Gunsmith attachment tuning

Beyond the platform feature that will be added to the Gunsmith, Modern Warfare 2 and Warzone 2 will also have an attachment tuning system. This is another feature that borrows from how stats have worked in the past few Call of Duty games and makes it more customizable.
As you use attachments and level up a platform, those attachments will get five different stat alterations. Each alteration will have a different set of pros and cons that can be fine-tuned to match a certain playstyle. Everything will come with power trade-offs, but one version may fit better with a platform than another.
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Featured image courtesy of Activision.