BP_GridComponent is used in SGK V2 to tell the system where to spawn grids. You can see them being used in may of the build part blueprints like the BP_WoodFoundation. In the BP_GridComponent you can set the sizes of the grid types like the image below shows.
Here you can set the size of all the grids in the system for each build part type. NOTE changing these values in the BP_GridComponent will effect all the grid sizes of that type in the whole project.
But if you select a BP_GridComponent in a build part blueprint you can enable UseCustomSize and then set the Custom Size variable to the size you want and it will only effect that specific grid component and the grid it spawns.
You can also set the Grid Type, this is the type of grid the component will spawn and build parts snap to. Check Radius is used to detect it a build part is at the grid components location. You likely wont have to change this value. But if your build part is large and doesn’t have collision near its pivot point then you may have to increase this value.
Attach Build Part means that any build part that is build while snapped to the grid this component spawns will be attached to the grid component, typical you should leave this a false (Unticked) but for things like the lock build part that needs to move with the door this should be enabled.
Snap Rotation Offset applies a rotation to the build part that is snapped to the grid that the grid component spawns. Typical you wont need to use this.