Are there any?
I would start by looking at Religious Orders that pray the Divine Office. They would most likely sing it using Gregorian Chant.
Some high Anglicans may. There might be some other “high” Protestants in Europe, but I would imagine that there are few in total.
Perhaps High-Church Anglicans and possibly High-Church Lutherans.
Yes, both. And the sad thing is that their congregations sing it - or chant it - much better than most catholic congregations!
Here is a clip of one Anglican monastic order that uses Gregorian Chant in their daily Office. My guess is that they use Anglican plainchant also.