Galvayne's Groove is a good way to help figure out a horse's age, ALONG with their dental stars. My trick is to take good photos of each side of their teeth, then from the front, and then to open their mouth enough to take pictures of their dental stars. Then I get the pictures on my computer, and compare them to the diagrams I have in some of my books (I'm sure something could be found online too). Of course, there are some horses that don't follow patterns, or have had some excessive wear or whatnot, but I have been very accurate when using that to age horses. I guess that wasn't quite your question, but hopefully it will help you find another way to age that gelding!