*ghedh-

The core idea of this root was:

unity, fitting together really well

From there, it slowly shifted from fitting toward the idea of good, which is also the only English relative I could find.
In German, it is also the origin of das Gitter, which is a sort of fence (fitting together) and of der Gatte and die Gattin, which are old school words for husband and wife, respectively – literally, the fitting one.