To be honest, it is hard to visualize what you are trying to do.
Hard to tell what "gun plate" means what to you. The Gun butt or"stock"?
Here is another example of extruding and cutting in Inventor. This shape is more like a wing.
Step1 Create sketch
Step2 Extrude with a taper
Step3 Create a work plane above the part
Step4 Create a sketch on the work plane
Step5 Extrude Cut the shape
Step6 the Results.
Now, that being said, all CAD software come with detailed how to's to get you started. If not, do a search online.