The Super GP-100 is built on the Super Redhawk frame IIRC. I would expect there the be nearly 100% compatibility, but it may not be stated due to the low production numbers.
I shoot my Super GP-100 in USPSA competition. (Thanks Kevin!)
I use a Comp-Tac kydex L-frame one. I did cut off part of the front sight hood to allow for quicker draws. And I added a block to offset the bottom of the holster to where it doesn't allow the barrel to point inward towards my leg.
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