Why does this project have both?
Very confusing.
I guessed this project is maintained with maven.
Let's delete gradle files.

Or If necessary, I will create PR about this issue.

I use maven. Diemex converted this to gradle a while ago, and I left it there since it doesn't affect me using maven at all. But I guess I could rename it away or something if it's that confusing - just thought leaving it there would let people choose how they want to build.

If gradle files are left for people who use gradle, the build script should be also updated.
Gradle build script for this project is outdated.

(In my personal opinion, you do not have to think about Diemex.
He isn't active in this project for 4 years. )

Makes sense, if the build scripts no longer work then ya prolly makes sense to remove them then.