The short version is:
How would I create a new version of the Gipon Prudensco, with the same effects/enchantments of the Sharpshooter's Garb, and place it in a shop in Port Maje?
following peardox's first mod guide, create a new directory for your mod and a new .gamedatabundle file inside it.
In the game files, find Gipon Prudensco in items.gamedatabundle. Copy all the data into your gamedatabundle file.
Use the online UUID generator to replace the item ID (you can also just come up with a UUID yourself but don't use any letters after F)
Find the sharpshooter's garb in items.gamedatabundle. Copy everything in the square brackets after "ItemMods"- an itemmod is what the game calls an enchantment, or any property of an item.
Go back to your gamedatabundle and replace everything in the square brackets after "ItemMods" with what you just copied.
Voila, a new version of the Gipon Prudensco, with the same effects/enchantments of the Sharpshooter's Garb.
This doesn't include upgrades though. If you want to add those, search for the names of the enchantments you want in items.gamedatabundle, copy their IDs, and paste them into the ItemMods square brackets.
To add to a vendor, follow peardox's guide in the First Mod thread