I have checked the source and one of the place that downloads plugin does not have this m2override defined. 

String m2Override = System.getProperty("apiman.gateway.m2-repository-path"); 

 I think Apiman gateway looks at io.apiman.gateway.engine.impl.DefaultPluginRegistry.loadPlugin(PluginCoordinates, IAsyncResultHandler<Plugin>) 

where as Apiman manager UI reads from 

 io.apiman.manager.api.core.plugin.AbstractPluginRegistry.downloadPlugin(File, PluginCoordinates)

This makes the application inconsistent. Please let me know if I can raise JIRA or pull request ? Thanks.

Best regards

Balu