gimp native batch convert?

[Ofnuts]

Now you are in Windows territory...  you have two problems:

1. You appear to be running the "PowerShell" and not the classic (aka "DOS" shell). Maybe the PS is compatible enough, but the command is crafted to navigate the intricacies of the quoting syntax in both the shell and the Python language, so it the shell syntax is bit different this could break things.
   
2. In either shell, when you issue a command (gimp in this case) it searches a gimp.exe in a set of known directories. But these are known to the system because they have been added to the PATH environment variable. An alternative is to specify the full path to the executable (which is fraught with peril since MS had the great idea to use a name with spaces in it, so you have to add more quotes).
   

This is about as far as I can get for you. To go further I'll have to make screenshots, which means installing Windows somewhere... and nope.