latest is based on the latest Fedora version, so right now it’s 40. gts is based on (latest)-1, so right now it’s 39. In the future, when Fedora 41 is released, latest will be upgraded to 41, whereas gts will be upgraded to 40.
It’s intended to ensure gts users experience slower updates and more tested, stable+solid system by not being on the most recent Fedora version. So if you prefer slower and/or less updates, but don’t mind having slightly “older” system packages, gts would be the better choice. On the other hand, if you prefer to have newer packages and get updates as soon as they come out following Fedora 40, you would want to use latest instead.