There are a few things you need to configure to make standard UE4 levels work in Gladius.
Save the level into your plugin folder: Click File>SaveCurrent and type a level name.
Open the Definition file again, type the name of your weapon blueprint file (e.g. MyLevel in this example), and select ModType = Level.
Click "Compile", then "Save", and finally close the definition file. You don't need to modify this file again.
There are two things that you need to modify to make the new level work.
Player start position: Move the player start actor to the ground level
Change all lights to and atmosferic fog to static: While this is not estrictly necessary I highly recommend it to achieve the 72fps framerate.