unable to run gems using custom Ruby version on macOS Mojave - ruby

Question

I get error when trying to run any gem installed using ruby from brew install ruby
Here is slather example but this is same for any gem I tried (Jekyll, CocoaPods, Nokogiri etc.)
/Library/Ruby/Site/2.3.0/rubygems.rb:275:in `find_spec_for_exe': can't find gem slather (>= 0.a) with executable slather (Gem::GemNotFoundException)
from /Library/Ruby/Site/2.3.0/rubygems.rb:252:in `bin_path'
from /usr/local/bin/slather:22:in `<main>'
gem is installed properly but for unknown reason ruby tries to use paths for system version.
I have read to not use rvm and rbenv and only brew version but this also do not work as above
ruby -v: ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-darwin18]
gem env:
RubyGems Environment:
- RUBYGEMS VERSION: 3.1.2
- RUBY VERSION: 2.6.5 (2019-10-01 patchlevel 114) [x86_64-darwin18]
- INSTALLATION DIRECTORY: /usr/local/lib/ruby/gems/2.6.0
- USER INSTALLATION DIRECTORY: /Users/[user]/.gem/ruby/2.6.0
- RUBY EXECUTABLE: /usr/local/opt/ruby/bin/ruby
- GIT EXECUTABLE: /usr/bin/git
- EXECUTABLE DIRECTORY: /usr/local/lib/ruby/gems/2.6.0/bin
- SPEC CACHE DIRECTORY: /Users/[user]/.gem/specs
- SYSTEM CONFIGURATION DIRECTORY: /usr/local/Cellar/ruby/2.6.5/etc
- RUBYGEMS PLATFORMS:
- ruby
- x86_64-darwin-18
- GEM PATHS:
- /usr/local/lib/ruby/gems/2.6.0
- /Users/[user]/.gem/ruby/2.6.0
- /usr/local/Cellar/ruby/2.6.5/lib/ruby/gems/2.6.0
- GEM CONFIGURATION:
- :update_sources => true
- :verbose => true
- :backtrace => false
- :bulk_threshold => 1000
- REMOTE SOURCES:
- https://rubygems.org/
- SHELL PATH:
- /usr/local/opt/ruby/bin
- /Users/[user]/.fastlane/bin
- /Users/[user]/.fastlane/bin
- /usr/local/bin
- /usr/bin
- /bin
- /usr/sbin
- /sbin
- /opt/X11/bin
any help appreciated