I teach English GCSE and have taught OMAM and Mockingbird. I love Steinbeck and find his message particularly relevant in today's socio-economic climate. OMAM works well and I'm surprised by how much the kids enjoy it. Lower ability kids can cope with it due to its brevity and the terse simplicity of the language used. For high ability kids, Steinbeck's use of prose, his narrative style and the political themes in the novel, give them plenty to get their teeth into. The fact that Gove cannot see the value if OMAM reflects on his ignorance; it's a stunning piece of prose and I don't think there are many better writers than him.
Japan in answer to your point: the current GCSE is obsessed with close language analysis to the point where sometimes, it becomes divorced from the overarching point of the novel. For students to access anything above a C-grade, they need to analyse language. For most students this is a very counter-intuitive way of looking at writing and it is true that it can make you look at style mire than content. I try hard to maintain a balance but it can get skewed. That's why writers like Duffy work well - she is very stylised and to divorce what she says from how she says it, is to miss the point. However with Harper Lee or Steinbeck, it's all about the message so some contorting is required to meet the grade criteria.
One thing is for sure: as an English teacher you have to work beyond requirements to inspire both a love of literature and to equip kids with skills necessary for the GCSE. The two do not work in synchronicity.
Gove is an idiot who is making the very self-indulgent mistake of drawing from his own experience and extrapolating universal truths. He could do with re-reading Mockingbird and taking on board some of Atticus Finch's advice. Reading Victorian novels when you're dyslexic, of below average intelligence, helping care for younger siblings and when the height of your cultural experience is The Voice is of benefit to no one. This will just further alienate struggling kids but maybe that's the point: let the peasants know their place, make sure that the education system teaches them that they have nothing of value to contribute - let the boys from Eton look after all the power and the money...