Sponge now has a Maven archetype to create plugins! For those not familiar with the concept of an archetype, it is basically a template that creates a maven project with a certain structure – in this case, the structure for a very basic Sponge plugin.
The Git repository is at GitHub - SpongePowered/sponge-plugin-archetype, and the readme there contains usage instructions. We are working on setting up a deployment of the archetype to the Sponge repository, and will provide an update when that happens.
While Sponge itself is built with Gradle, Sponge does not actively try to prevent developers from using other build systems. A lot of developers coming from Bukkit are familiar with Maven, and this archetype will hopefully allow those developers to get started more quickly.