gem help だと全部のサブコマンドが出せない!のでワンライナーメモ。ただこれだと標準組み込みのコマンドのみなので注意。
ruby -rrubygems/command_manager -e 'puts Gem::CommandManager::BUILTIN_COMMANDS.join("\n")'
build
cert
check
cleanup
contents
dependency
environment
fetch
generate_index
help
install
list
lock
mirror
open
outdated
owner
pristine
push
query
rdoc
search
server
sources
specification
stale
uninstall
unpack
update
which
yank