Was Gerald punishing Gudrun?............

Hmmm... I think Gerald does show Gudrun a form of affection. For his time and class - he was brought up in a 'stiff upper lip' climate, public school etc, and of course with a dysfunctional, coldly reserved family, more so it would seem than people like Birkin - he is actually being very passionate. Gudrun responds to it. We do feel sympathy for him, he is grieving etc, but also as an old-fashioned person he is literally dying out. His way of doing things can't go on, it won't survive.

But there is a definite point when things break apart, when they admit they don't 'love'; they are attracted to each other, but they are not being 'loving'. Gerald admits his feelings are conflicted, that he hates her as well as loves her. I think Gudrun senses this and tries to educate him, 'love me more and want me less', but to do so she becomes manipulative and false. As she goes on to describe his treatment of her, it is an exaggerated list of his faults, but without the 'get out' of the angry yelling of Ursula. Gerald simply accepts it at face value 'Horrible to you?' without fighting back verbally as he might have done if their mutual love for each other had been accepted. Instead, his abuse comes later, as more of a 'punishment' for Gudrun for going back on her word about his being horrible, and for still making him want her even when she has no respect or affection for him, and is therefore equally horrible. You are right, it is tortuous.