If the GPU is checked, the drivers are sufficiently up-to-date, and the scene fits into its memory then the GPU will be used 9and the CPU, if that is checked). Try a really simple scene (just Create>New Primitive and add any you like with its default values) - that should render really quickly, so after it's done go to Help>Troubleshooting>View Log File, scroll to the end and in the last few lines there should be a summary of which things did what to the render - was the GPU used according to that?

Do bear in mind that, by default, Windows Task Manager will not show the Iray usage - if you go to the Performance tab and set one of the graphs for the GPU to show CUDA (or Compute 0 if there's no option for CUDA) you will get a more accurate result.