On latest version 1.6.0-163 it's not possible to install additional fluent addons with fluent-gem. Looks like an issue within https://github.com/Microsoft/OMS-Agent-for-Linux itself as the latest version states in it's release information:
Fix for broken gem and fluent-gem commands in previous release
I've created a change to incorporate 1.6.1-3 in this repository, PR is incoming.