Apparently most shampoo bars aren't CGM approved (because of the soapification process) but I've had the most success using nothing but a shampoo bar made of bentonite clay and coconut oil :
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Amazon.com : Naturel Relax Moisturizing Shampoo Bar and Conditioner Bar Set, Shampoo Bar and Conditioner Bar for Curly, Wavy, Dry, or Frizzy Hair, Vegan & Cruelty Free, Zero Waste, Made in USA :