Right now, the Unity connector will not attach SpeckleProperties for raw geometry types (Meshes, Breps etc…)
But it will attach for object types that contain geometry (under a displayValue property) e.g. Walls, Floors from Revit.
But you can also send these from grasshopper by wrapping the geometry in a base object, with your custom props using the Create Speckle Object node:
Just make sure the displayValue property is a detached list, where you can add your mesh/meshes. And then you can add as many custom properties to the object as you’d like!.
The result : Speckle
Unfortunately, in Grasshopper, you’ll need to convert the brep to a mesh to perform this workflow. But if your goal is GH → Unity, this should actually give you more control over how the brep is meshified.