Kindle Previewer
Kindle Previewer is a free desktop application provided by Amazon for previewing how e-books will appear when read on a Kindle device or on a tablet or smartphone that includes a Kindle app. The current major version as of the last update of this web page is Kindle Previewer 3.
You may download Kindle Previewer from the Kindle Previewer page on Amazon.com.
When Gedcom Publisher starts, it will attempt to find Kindle Previewer and add its .exe file as an item on the File > Browse Book submenu.
The installer for Kindle Previewer 3 installs the program in an unusual location under the AppData folder. Gedcom Publisher attempts to locate it there. If Kindle Previewer is installed in a location that Gedcom Publisher does not search, you may add Kindle Previewer as a Viewer tool as described on the Tools page. The correct .exe file is named Kindle Previewer 3.exe. Set the Accepts property to "MOBI".
Using Kindle Previewer 3 with Gedcom Publisher
When you start Kindle Previewer 3 via an item in Gedcom Publisher's File > Browse Book submenu, Kindle Previewer 3 will open the MOBI version of your book and convert it to Kindle format. That process will take some time. When the conversion is complete, Kindle Previewer 3 will open the Kindle version of the book. Browse the book to see how it will appear when rendered on various Kindle devices as chosen from the Device Type menu in the main Kindle Previewer window.
You may use the File > Open... command in Kindle Previewer 3 to open .epub and .mobi files, as well as other file types.
If you use the File > Open... command to open a .mobi file written by KindleGen via Gedcom Publisher's File > Convert Book > KindleGen command, Kindle Previewer 3 will open its Book Conversion status window and re-convert the book even though conversion should not be necessary. I am not sure why that happens, but it is unrelated to Gedcom Publisher.
After opening a file in Kindle Previewer 3, and waiting for the conversion to complete, you may use its File > Export command to save the book in MOBI format or AZK format.
For instructions on using the AZK format, see the Kindle (iOS) page.
I do not recommend using Kindle Previewer 3 to create MOBI files: the version of KindleGen that is included in the Kindle Previewer 3 install file appears to be out-of-date with respect to the version of KindleGen available as a standalone program from Amazon.