Version 5.7 is a smaller scope, mostly a maintenance release, that still brings few new features to the meta fields and boxes system, various improvements and most of the JS libraries updated.
Select based meta fields have been improved with two new options. One is to show or hide first empty value, and the other is mass edit mode for the list of values using the plain textarea, that can be very useful to add a large number of values at the same time.
The plugin also has a new special meta field that is available only inside the meta box editor (Special -> Information). This field allows you to insert some information to display inside the metabox as a field, but on the front end, the metabox object will not contain this field, and it saves no data. This field supports the use of a filter, so in code, you can modify the information displayed by the field.
GD Content Tools Pro will get one more major version by the end of the year, with some new meta field types. If you find any issues with the plugin, stop by the forums and let me know.