I had a Gam antenna on my boat until recently. I ran into an issue with the GTO-15 wicking up moisture. The GTO-15 corroded and I had to send the antenna back just to get it replaced. Not a great option when you are out and away from the dock on an extended trip. And I'm not the only one that I heard of with this issue.
Two comments to be fair. First, the antenna worked just the way it was advertised. It worked just as good as my new backstay antenna. Second, the customer service was outstanding. They fixed and returned the antenna within 10 days. As long as you have a single backstay, the antenna is easy to install.