I'd assume that the 'current directory's is not where gradle is stored when you try to run gradle.
If you go the folder where gradle.bat is, hold shift and right click, there should be an option to 'Open command prompt here'. That will set the directory to the correct folder.
Alternatively, you could use command prompt 's command to change directory. That would be 'cd <Wherever gradle.bat is>'.. eg 'cd C:\\Minecraft Modding\Forge'
Hope this helps.
Talen