I use Groovy editor from Groovy-Eclipse plugin to edit Gradle files. Since Gradle is basically Groovy it works very well. The plugin is available in the Eclipse Marketplace and GitHub.
Buildship does not support source highlighting and probably never will due to planned transition from Groovy to Kotlin.
There is the following post in their forum: "this is planned to be supported by Buildship at some point in the future. But, we do not have this on our immediate road map".