So now that you have a matrix on the stack what are you going to do with it?? gcalcRPN gives you many choices. If you have 2 matrices on the stack you can add or subtract them just with the add or subtract buttons (as long as they are the same size). You can mautiply 2 matrices if they are of proper size, and you can multiply a matrix by a scalar by putting a scalar (real number) on teh stack and pressing multiply. You can also multiply a matrix by a vector to get another vector but we haven't gotten that far yet (The next section in on vectors).
There are other matrix operations that you can choose also. Remember when you put the matrix on the stack and you had the choice to create a new matrix or preform a matrix operation.
You can choose the matrix operations button to see a list of matrix operations that you can use. (Actually, at the time of this writing I haved't added that yet but it will be there soon!)
gcalcRPN is into giving you a choice, and you always have a choice of how to preform an operation using the func button.