Altar gongs are not like the Oriental type of flat gong with an atonal sound. These are something like the gong on an electric fire alarm bell, except heavier and mounted on an upright pedestal. Most of them have a solid and deep musical pitch, but a few of them sound like inverted steel mixing bowls. Some have three gongs, one above the other, to be struck sequentially (N-B-C!). I believe a factory in the UK still produces single altar gongs, as there is a nearby Episcopal church that has a fairly new one. I bought one on eBay and have used it for outdoor Masses where the sound can carry. The effect is very dignified and reverent.
There is no liturgical norm as to whether altar bells should or should not be muffled, but stopping a bell from resonating creates an ugly and non-musical effect. The nature of a bell is to resonate, rather than to make an abrupt sound like a Lenten clacker. That is why the better quality altar bells had legs to elevate them off the floor.
