All weapon mods need to be child blueprints of GladiusPhys_Mod, the parent weapon class in the game.
Go to Content>Blueprints>Mods and right click on GladiusPhys_Mod.
Select "Create Child Blueprint Class"
Type the name of your new weapon (e.g. MyWeapon) and then drag and drop it to your Plugin Content folder. Select "Move".
After this step your Mod folder should look like this:
Open the Definition file again, type the name of your weapon blueprint file (e.g. MyWeapon in this example), and select ModType = Weapon.
Click "Compile", then "Save", and finally close the definition file. You don't need to modify this file again.
Double click on your weapon file to open it. The first time you open the file you will see the simplified editor, click on "Open Full Blueprint Editor" to get access to all the features.
