3DGS is very complicated to have good lightening , and complicated to use.
First check in MED object properties :
Diffuse
Ambient
Specular
Then adjust Sun and Ambient properties of your Map.
They have a lot of influence if you don't use any shader.
I also would recommend you to use some shader, they are a good bunch ready to use.
So what you have to do is try and experiment.
this has nothing to do with 3dgs at all. it has to do with the way 3d models are shaded in realtime in just about any engine. its the default dx shading pipeline, same thing with ogl.
