What GSM should I look for in a microfiber towel?

It depends on the job. Reviewers across detailing communities suggest 300-400 GSM for polish, wax, and sealant removal (shorter nap leaves less residue), 500-600 GSM for general buffing and quick-detailer wiping, and 800-1200+ GSM dedicated waffle or twist-pile towels for actually drying the car. A single GSM doesn't do everything well.