May I add a listing if it is co-listed with someone who is not a GNMLS subscriber?

No; listings co-listed with non-subscribing agents are ineligible for entry in GNMLS and are subject to a fine of $500 and prompt removal. Only listings that solely involve GNMLS subscribers, and the only co-listers allowed in GNMLS are other GNMLS subscribers. See GNMLS Policies & Procedures Section (V)(D) on page 6.