I have not sailed on one but I have observed them under way. The do not appear to be especially tender but they don''t point very well. It seems to me that I have seen both gaff and junk rigged versions. The junk rig version in particular sticks in my mind as pointing particularly badly and making huge gobs of leeway.
They have always struck me as a good boat for a slow reach up into the Arctic Circle to get iced in for the winter, but with minimal ventilation, a low weather deck ht, a high center of gravity, and comparatively small accomodations for their size and weight, not much use besides that.
The most recent Colvin Gazelle that I observed underway I beleive was called something like Surrender and I think that she is still for sale in Annapolis, Maryland.