Hi Michael,
Thank you for your time.
I found the solution to my problem, posting it here for the people who might run into the same difficulties.
This tutorial by Muffin Man gave me the solution (close to yours actually). I added a gems/bin path to my zsrhc file :
export PATH="/usr/local/opt/ruby/bin:/usr/local/lib/ruby/gems/2.6.0/bin/:$PATH
I then had multiple problems of out-of-date gems - bundle update took care of them.
thanx again Michael 