A version string like that indicates that at build time, one more commit was included after the rc tag. What that is, only the package producer will know.
That text just means “this build includes the next commit after your rc” in syncthing. It’s nothing special. the `g` in `gb7b494b7` basically simply stands for “git” therefore searching github with it won’t work straight.
It appears strange, but it’s safe since your update manager is detecting the most recent main build after 2.0.13~rc.1. Check the commit hash `b7b494b` in syncthing’s repository to see the precise changes.