You want to make the gem command available from any folder on your MacOS ? Here is what you can do
Tested Configuration:
MacOS: Sierra 10.12
gem: 2.7.4

1. Go to a specific folder

Go to your terminal, and type cd /usr/local/bin

2. Check if gem is already installed

Type gem -v

• it should prompt some numbers, like 2.7.4. Now, go to 3.

• Otherwise install gem. I recommend fully installing Ruby by typing brew install ruby. You can skip 3. since gem should be available everywhere

3. Make it available

Type sudo nano ~/.bash_profile
Type your password when prompted

Then add these lines at the end of the opened file
export PATH=/usr/local/lib/ruby/gems/2.0.0/bin:$PATH
export PATH=/usr/local/opt/ruby20/bin:$PATH

And exit with ctrl-c then y

For further reading, go to stackoverflow