[UPDATED]
mvnrepository.com isn't a repository itself, it's a search engine for Maven artifacts.
By looking at the "Download jar hyperlink", I see that the liferay jars are available in Maven Central http://repo1.maven.org/maven2/com/liferay/portal/.
In the repositories section of your BuildConfig.groovy, make sure that the following line is there
mavenCentral()
I think the portal-impl
artifact contains com.liferay.model.user.* related classes which are what you're looking for.
In that case you would have the following dependency in your BuildConfig.groovy
compile 'com.liferay.portal:portal-impl:5.2.3'
If you require additional liferay classes, assuming you have liferay running somewhere, you could search the jars for a specific class name (http://java.net/projects/jarscan). Once you know the jar name, you can search it on mvnrepository.com
and add the relevant dependency to your BuildConfig.groovy
.
Hope it helps.
You can reference liferay dependencies in your BuildConfig.groovy. You would need to confirm the repository URL and then it should be fine.
http://mvnrepository.com/artifact/com.liferay.portal
You could exclude those dependencies from packaging.