Answer from the GeneratePress support:
OK, i think the plugin developer needs to take a look at the issue.
The Theme and GB uses core functions to return the author name which uses: get_the_author
And this snippet here uses the get_the_author_meta('display_name')
Both are functions that the plugin developer said should work.
And neither GP or GB interferes with those functions.
So unless another code is interfering with the result then there should be no reason it does not work.
Ask the developer to take a closer look, or ask them where is the submitted author stored, eg. in the post meta, as it may be easier to display that.