Is your OGM 2D or 3D?

If it's 2D, you might need to touch it up with an image manipulation software (e.g. GIMP) such as scaling it to the correct size. Then you can use a CAD software (e.g. FreeCAD) to extrude it in the z direction to create a 3D model. Afterwards, you can export the 3D model as .stl or .dae and load it in your gazebo .world file. A nice example can be found from amazon (https://github.com/aws-robotics/aws-robomaker-small-house-world/blob/ros1/worlds/small_house.world).

If you're OGM is already in 3D, you can probably convert it to .stl or .dae and use it directly. But you may need to clean it up or do some preprocessing (e.g. removing noise, inflating the points, etc.) to make it a better model