Hi AT (@TAshish),
you should be able to get the COG of the platform with respect to the GCS out of your CAD system. This can be seen in the example below.
I took a quick look at your simulation set up. I can see that your welded members are modeled as separate bodies. I prefer to join welded bodies in my CAD software prior to bringing them into SimScale. If you wish to use separate bodies I suggest that your bonded connections (5 to 8) be defined with a position tolerance. See this discussion for details.
For more information on preparing geometry for simulation you can watch this video.
Regards, Ben