Actually, I don't get what's wrong with using sudo for the use-case of installing an executable system-wide? In principle that's what every installer does, even the MacOS-installer when installing system-wide applications. The only other option would be to install system-wide apps in a directory where every user has write access - a situation I'd find far more problematic.

However, if you'd like to install the gem just for yourself set the proper environment variables and install to your home folder.