Hi Matt,
It’s not possible to add it the normal way w/ CSS border property.
Were you setting up a border for a GB shape? If yes, then you can duplicate the same GB shape on its container block and set its CSS to be off-position by a few pixel so it serves as a “border”.
Example: https://share.getcloudapp.com/GGu4mb0R
The “black border” you see in this image is actually a duplicate of the red shape. I just moved it a bit upward by a few pixels so it appears like a “border”.