Steps to reproduce the issue

You have to have a JSON error in the GSD data. In my case it was something with Unicode.

Expected result

Print the JSON error between the script tags like:
JSON error: Some JSON error message

Actual result

You get an PHP error: Array to string conversion

System information (as much as possible)

Joomla Version: 4.2.6
PHP Version: 7.4.8

Additional comments

The problem is in administrator/components/com_gsd/GSD/Json.php, in the method generate():

if (json_last_error() !== JSON_ERROR_NONE)
{
    $json_string = \JText::sprintf('JSON Error: %s.', json_last_error_msg()) . ' ' . $content;
}

The variable $content is the array, which couldn't be encoded to a JSON string. This can not be concated with the error message, and should be omitted.