Having the 2.0.3 GWT plugin installed
Never having used the 1.5 GWT plugin
I was getting the following error when clicking the GWT compile button :
The solution was to move my GWT dependencies above my Maven dependencies.
Projects using GWT 1.5 or lower may only specify one entry point module to compile
Right click on the project -> Build Path -> Configure Build Path... -> Order and Export
Click on GWT SDK and Up until it is just below the JDK.