"It is implied by GCT's ads that the trips are designed for senior citizens. However, each trip does indicate your must be in good/excellent physical condition. Not all senior citizens are in good/excellent physical condition"
I'm amazed that you would book a trip without taking the insurance when you knew that your husband was not in good/excellent physical condition. I read the physical requirements before booking a trip and find them to be quite clear -- a lot of walking, standing, climbing stairs etc. sometimes over rough terrain -- and decide whether I'm fit enough before booking.
Since you had problems on the Danube cruise I wonder why you didn't take insurance for the Costa Rica trip. Even if you'd decided against the GCT insurance you could have bought insurance on Insure My Trip up to shortly before your departure.
I'm sorry that you lost money but you need to be realistic about traveling now.