Not sure where you're located but where I am G. 'Biokovo' is a zero-maintenance perennial that requires healthy soil and not much else. I also have G. 'Rozanne' along with one other whose cultivar name escapes me at the moment. These plants thrive on neglect in my garden--I do nothing to any of them and they've been happily growing here 3-5 years.

I checked my perennial reference book ('Perennials for Every Purpose' by Larry Hodgson) and pruning isn't mentioned in reference to these plants.

Only cut back if they are over growing an area and then they can take any amount of it.

I usually don't do anything, maybe deadhead the flowers at most. Last year I cut the foliage way back in midsummer to see if it would make them look tidier. They did look nice and neat once they grew back, but it took a while for them to rebound, and in the meantime they looked pretty ratty. I don't think it is worth the effort unless they are really sprawly. You might experiment to see what works for you. They can take the abuse.

I cut mine back last year. The foliage was looking ratty after a few dry spells, so in August sometime, maybe, I just cut off all the ugly foliage which was most of it. Believe it or not, the leaves came right back, healthy and clean looking going into the fall. This spring, they are looking better than any spring I've seen them. So I feel it was a good thing. I will probably do it again this year. They didn't need it right after flowering though, it was later in the summer. I would do it anytime the plant was looking worse for wear.