bundle gem gem_name --test=minitest
It is not currently possible.
-t, --test=minitest, --test=rspec
Specify the test framework that Bundler should use when generating the project. Acceptable values are minitest and rspec. The GEM_NAME.gemspec will be configured and a skeleton test/spec directory will be created based on this option. If this option is unspecified, an interactive prompt will be displayed and the answer will be saved in Bundler's global config for future bundle gem use.
bundle gem generator generates less than 10 lines of tests, so converting to
MiniTest::Spec is hardly a herculean task.