Ruby's Debugger seems to have improved with Ruby 3.1.
See this README for details.

Debug Gems

Set environment variable EDITOR like "EDITOR=code" or "EDITOR=vim" then if we run bundle open <gem_name>, it will open up our editor right to the directory of the installed gem itself. (From Ruby Weekly Newsletter)

Useful Gems

It is too early for me to make my own list (As of Jan 2022)
But this post has good list (I think)

As of Feb 2022, I've started adding my own list 😆. Seriously, the above list is still valid.
I just wanted to start my own list of Gems that I "may" need in the future.