GNU grep is not multi-threaded.
Since you chose to be cute, I'm going to take a guess that you think this means the comparison between the tools is unfair. If that's an accurate interpretation, then I will say that your question is irrelevant because ripgrep is faster or as fast as GNU grep on benchmarks using a single thread too. In fact, that's the whole point! It says so right in the project's README.
http://blog.burntsushi.net/ripgrep/#single-file-benchmarks