Sphere filter in Gimp but not in CLI??

Some filters do not have a reference line. However, if you see them in GIMP, you may use them in the CLI. Do the following to discover the proper command.

Or, go to the following and examine the code:
https://raw.githubusercontent.com/dtschump/gmic/master/src/gmic_stdlib.gmic
https://github.com/dtschump/gmic-community/tree/master/include