Hi Michael,
Spheres is designed to work as projects with a structure and a config.scm file. The tasks will handle all that dependencies for you in different situations.
Using gsi directly is not supported currently. The reason is that you are not supposed to write ##import code in your source files, but rather use the config.scm files to define direct dependencies and let the different tasks to do the work for you. Maybe future releases address that usecase, for individual source files without config.scm.
If you want to run interpreted code you need to:
"sake host:run"
inside a project.
Otherwise, to compile use:
"sake host:compile"
So, if you want to run code as gsi, just use the "minimal" skeleton and place the code in the main.scm file.
Cheers!