My work around solution:
rack-cols and proxy api request with heroku.
=> request: https://rubygems.org/api/v1/gems/tachikoma.json
=> response with CORS params.
Issue which I added was duplicated 😉
CORS(Cross-Origin Resource Sharing) support? · Issue #650 · rubygems/rubygems.org https://github.com/rubygems/rubygems.org/issues/650