It depends where you need the code, if you’re using a function to handle the conversion from GF to ACF then this is where you use the code.
How are you handling the GF to ACF part?
It may be you need a GF function ( gform_after_submission() ) for that form, see here for examples
But you can then convert the GF field using the above and that should update your ACF date field with the right format!
