I hope you get a better answer than this, but here goes
Infinite gore means sewing together many triangular pieces, and I can't help you there, but here's a link that might help
Infinite Gore Skirt Instructions with PhotosIf you want to use a single piece of fabric cut into a semi-circle, you will need a piece of fabric 65 inches wide by 130 inches (4 and two-thirds yards) long. To both dimensions, add some for a hem appropriate to the fabric or limited by the widths available. Not many fabrics come that wide. I don't know if you can find a curtain with those dimensions unless you sew several panels together.
These sites might be useful:
Circular CloaksSemi-Circular Cloak PatternMore simple cloaks - no measurementsGood luck and please let us know how it works out!