One of my first great managers at AMS tought me the Keep it Simple Stupid (KISS) principle. This is a lesson software engineers often forget. Often those that consider themselves experts fall into the trap of not following the KISS principle.
For example, 2 of us spend 3 hours troubleshooting an issue with one of our applications last night. Both of us have 17+ years epxerience in software development. The application we were working on was built by a "Ruby on Rails" expert. This expert is a very good developer and writes very good code and maintained a good suite of tests. One mistake the expert made was not following the KISS principle.
So in the end we found the problem and fixed it, but it took probably 10x longer because of the complexity of the application.
Just because there is a gem for everything doesnt mean you should use it. Try to keep your applications as simple as possible, make sure you really need a gem before using it. If its doing something to modify the way things normally work, then document it throughly. Think about the developers that will be maintaing your application for the next 10 years. Will they find your code simple and straight forward? How will your code hold up to version changes and dependency changes?
Dont forget to KISS!