This link is on the SS site but it appears you must first have a plan from a participating carrier
https://tools.silversneakers.com/Eligibility/CheckEligibility
Finally found the right link............had to call Silver Sneakers to find it.
https://tools.silversneakers.com/Eligibility/HealthPlans
Pretty much confirms no one with a Plan G has it........Humana/UHC/BCBS offer SS, but do not have a G at this time.