Speaking as one who visits lots of semi-natural habitats where gorse is native in the course of work, I don't think I've ever seen Gorse growing successfully as an understorey to anything. I just can't conjure up an image of it. It forms thickets of itself. Maybe under birch, but birch is a pioneer, the gorse was probably there first.
And I have read that walnut is very unfriendly to other plants.
I only once found gorse as a understory and that was in a designed environment. It was under a trimmed sycamore. The advantage of gorse is that it dies down in the middle and tends too grow outwards. Its a nitrogen fixer and has flowers of witch you could make a wine. Goats eat the young shoots. That's where my knowledge stops. This question is more fore the European forum members I think.
Gorse tends to prefer dry sandy soils on the low pH side. It also loves sunshine, if you go by it on a really hot day you can hear the seed pods crackling, and it smells like coconut oil. It responds well to trimming but is very painful to tread on afterwards. We have a saying here "when gorse is in flower, kissing's in season". In other words it can flower at any time of year.