When I use Maven API search function it works fine with dependencies in Central repo (e.g. retrofit), however, it doesn't retrieve dependencies from Google repo (e.g. play-services-plus). Even though they both available in the web search

To programmatically search 'Google's Maven Repository', you can search the group-index.xml for the artifact you are looking for via https://maven.google.com/group_path/group-index.xml

So in your case you would search:

https://maven.google.com/com/google/android/gms/group-index.xml

To get the release data (pom or aar) for your chosen artifact/version, you can do a GET using the following syntax: https://maven.google.com/group_path/library/version/library-version.ext (where ext is pom or aar)

So in your case you would do a GET for:

https://maven.google.com/com/google/android/gms/play-services-plus/16.0.0/play-services-plus-16.0.0.pom

or

https://maven.google.com/com/google/android/gms/play-services-plus/16.0.0/play-services-plus-16.0.0.aar