It's not always clear whether an adjective is gradable or not and sometimes you will find usage that goes against the rules taught, but ask yourself whether something can be measured- you can measure steepness, so it's gradable. If it's a clear yes/no question, then it normally shouldn't be gradable- something either blows your mind or it doesn't.
We would probably agree that a very steep hill is steep, but we may disagree around the middle- you might say a hill is steep but I may say it's not that steep, but we agree that we are talking about an incline. However, you might think a film is mind-blowing, but if I disagree, it's not a question of degree- I simply didn't find it that good- you say yes and I say no.
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