Multi-body Design is not intended to duplicate an Assembly file. My suggestion would be to complete your frame in the part as multi-body without the gussets and then publish your multi-body using the Assembly file option and then in the generated assembly, add your gusset part where needed. Then you can get your BOM.
Another option would be to use the Frame tool in Assembly. It is going to be much more efficient than trying to use Multi-body in Part, and it will provide a cut lengths for your frame members in the BOM.
You still appear to be confusing the multibody process with assembly.
If you have a bracket that has 2 identical side plates, you would model just one of them.
If we then assume you have a base plate for the bracket you would then model that in a separate part file.
Now you create an assembly file and place the base plate and 2 instances of the side plate.
That is the only way to get a correct BOM.
If your side plates are mirrored versions of each other you can create one, then in a new part file Insert Part Copy of the first, with the mirrored option.