If a Global Style is deleted, any blocks that were assigned that style become unavailable in the post editor. The block displays the message:
“This block has encountered an error and cannot be previewed.”
The block will still display on the front end, but without any styling. Unfortunately, the block becomes un-editable and any content inside it is lost.
I’m not sure if it’s possible, but it would be great if GB detected the missing Global Style, reset the block to default, and kept the block and its content editable. Perhaps with a message warning that a previously applied Global Style couldn’t be found.