I'm not seeing it in the history, or across any other modules.
```
$ grep -R --color -n "islandora_book_gs" sites/all/modules
./islandora_paged_content/includes/admin.form.inc:127:  return $form['local_derivative_settings']['pdf_derivative_settings']['islandora_book_gs'];
./islandora_paged_content/includes/utilities.inc:349:  $gs = variable_get('islandora_book_gs', '/usr/bin/gs');
./islandora_paged_content/includes/utilities.inc:385:  $gs = variable_get('islandora_book_gs', '/usr/bin/gs');
```
```
[nruest@digital:islandora_paged_content] (git)-[7.x]-$ git grep -e "islandora_book_gs"
includes/admin.form.inc:  return $form['local_derivative_settings']['pdf_derivative_settings']['islandora_book_gs'];
includes/utilities.inc:  $gs = variable_get('islandora_book_gs', '/usr/bin/gs');
includes/utilities.inc:  $gs = variable_get('islandora_book_gs', '/usr/bin/gs');
```
...and [blame](https://github.com/Islandora/islandora_paged_content/blame/7.x/includes/admin.form.inc#L127) points to @daniel-dgi