I'd suggest a few trials at home before the viewing.
Try the following: Put your camera on Manual Mode with ISO of 400, Aperture of f5.6, and Shutter Speed of 1/125. Attach flash to camera and make sure it's set to ETTL. Do some trial shots with both close and more distant subjects, and see how you like the results. If your camera has an FEL setting, it might be worth setting that to +1/2 or +2/3stop, and again seeing how your results come out. If you can do similar trials in a space comparable in size to the gallery, all the better as the inverse square law reduces your flash strength very quickly.
Be on the lookout for reflections from mirrors and glass at your Private View. And if the gallery has a very high or dark ceiling, much of your flash output will disappear there.
(If you use the more automatic modes the camera will take over, and may not do what you want.)