I retried my setup:
apt-get install libguestfs-tools -y
Couldn't find the guestfs.rb and _guestfs.so files. Figured out the files were from
ruby-guestfs
apt-get install ruby-guestfs
Was able to find those files, added it in the load path
require "guestfs"
> LoadError: incompatible library version -
/usr/lib/x86_64-linux-gnu/ruby/vendor_ruby/2.5.0/_guestfs.so