That's because you're on windows 10. It tries to integrate its crappy powershell instead of good ol' command prompt. You can fix this with a registry hack, but its easier to select your folder in command prompt using the cd command. I usually type the gradlew commands separately.
Update: Seems to be working, surprisingly. All I did was add JAVA_HOME to the System Variables list, so thank you for pointing that out, couldn't have done it without you. (just imagine a happy face, cuz emojis are weird).
If you are skeptical, here is the output from the working command window:
... Is what I would say if it had gone smoothly. Sadly, there are more obstacles ahead. The command window does open, and does run, but during
"gradlew setupDecompWorkspace" it came up with an error. (gradlew eclipse ran flawlessly):