Install ctags in vim using home brew
brew install ctags
I have set up this mapping in my vimrc file for the ctags

map <C-F12> :!ctags -R --exclude=.git --exclude=logs --exclude=doc . <CR>

I just press

ctrl + fn + f12

and voilla I can use

ctrl + ]

to go the definition of function in a rails project.

The above did not find do tagging for gem specific functions.
I had to add

bundle list --paths

to the above line as well.
So now I have a new mapping which is something like this

map <C-F12> :!ctags -R --exclude=.git --exclude=logs --exclude=doc . $(bundle list --paths)<CR>

Now I go to a gem specific function and type

ctrl + ]

and voila I have migrated to the gem file and now I can have a look at the gem source.

useful links: https://blog.sensible.io/2014/05/09/supercharge-your-vim-into-ide-with-ctags.html
https://andrew.stwrt.ca/posts/vim-ctags/