To use GNAT, put its bin directory in front of your path in a DOS Window.
For example, assuming an installation under C:\GNATPRO\wave:

C:\>path C:\GNATPRO\wave\bin;%path%

C:\>gnatls -v

GNATLS <version> (<date>) Copyright 1997-200? Free Software Foundation, Inc.

Source Search Path:
   <Current_Directory>
   C:\GNATPRO\wave\lib\gcc-lib\pentium-mingw32msv\3.2.3\adainclude\


Object Search Path:
   <Current_Directory>
   C:\GNATPRO\wave\lib\gcc-lib\pentium-mingw32msv\3.2.3\adalib\

Project Search Path:
   <Current_Directory>
   C:\GNATPRO\wave\lib\gnat\

C:\>
