Not sure I get what gem not found is but the application I started from
scratch is still running okay when I start it using this command.
Rubygems is a package management system for ruby libraries. Packages
are called gems, so gem not found means you're missing a library.
Rails apps use bundler these days to manage their dependencies. A
freshly generated app is locked to the version of rails you have,
whereas the one you've downloaded is locked to whatever was on the
author's machine at the time (this is a good thing - it makes it less
likely that instructions won't work because of differing versions)
You can install all of an apps dependencies by running
From the root of the app