OK worked it out - I think!. You create a module and then bring it in to the module instance using the module id.
Which means the only difference between a module position and an instance is that you can style the module, and use it in as many configurations you choose, through an instance, without having to go to module manager ?
You can "pull" the output of a joomla module into the gantry 5 layout without the need to publish it into a position in the assigment settings of the module itself ...
You just need the id from a module in Module Manager: Modules.
Module needs to be published ...
you can install g4 templates and the g4 framework and also have g5 templates with the G5 framework.
But you can't use g4 templates in the g5 framework.
For module chromes check
docs.joomla.org/Standard_Module_Chromes
to get an idea ...
Right now Callisto has only one chrome ... but that might change in the future.