We have published the source code for the GeoDNS plug-in at GitHub: https://github.com/jhsoftware/sdns-GeoDNS.
The plug-in is released into the public domain as open source. You are most welcome to fork the source code, contribute code enhancements, and/or use the source code as a starting point for developing your own plug-ins.
For a full list of plug-ins available for Simple DNS Plus click here.
For details on developing plug-ins for Simple DNS Plus click here.