I applied through a recruiter. The first step is a 90-minute written test covering probability, statistics, basic finance, etc. I took this at a remote location. The exam is then scanned and transmitted to G-Research. If one passes the test, you move on to a phone interview and then potentially an in-person interview. I did not advance beyond the first step. It is certainly worthwhile to refresh one's knowledge, especially if years have passed since undergrad and you have not taken a test since then or early in grad school. If you don't, the test will probably be quite difficult.

The interview process was quite fast, I did an initial technical exam on math and coding, then had an another technical interview where they asked math, machine learning and programming question.