I installed data_mapper for a Sinatra project. Curious, why is it when I do
gem install brew
$ which brew
gem list to get the list of gems that are installed. If you can't find data_mapper, it may be that the gem name is different from what you expect.
Also, if you're just doing
which brew to find brew, you aren't finding the gem called brew, you're finding the location of the brew executable. Try
gem which brew instead.
If you're looking for data_mapper by doing
which data_mapper, you probably won't find it.
which is a unix program for finding unix executables, and data_mapper probably doesn't have one.
Since your goal is to verify a gem is installed with the correct version, use
gem list. You can limit to the specific gem by using
gem list data_mapper.
To verify that it's installed and working, you'll have to try to
require the gem and then use it in your code.