Are you using the latest Gemini IRAF 1.13? I’m having a hard time seeing from a quick look at gscut how this error would occur, so I think we’d need to see your data and the command you’re running to help out further. This may make a better helpdesk request than a forum post.
Great. Is this for GMOS South? Support was only added for the new GMOS-S detectors (installed last year) in the latest Gemini IRAF 1.13 release. Note that you can get both IRAF and the Gemini package easily by installing Ureka (http://ssb.stsci.edu/ureka/) if you need to do it again in future.