Mass GSX Lookup?

do a computer search based on some criteria and look at bottom - take action on results drop down has GSX lookup as option. I wouldn't try to do more than a few hundred at a time from personal experience.

indeed. take it in small chunks for the best results. If you ask it to update too many at once it might hang on you and never finish.
When I was doing a mass update I used JSS ID ranges as part of my search criteria, i.e. JSS ID from 1 to 300, etc.