The best thing to do in a conclusion for an essay, whatever the topic, is to restate your main points and then find some way of ending in a way that makes the point of your essay clear to any reader. A conclusion is not a time or a place to introduce anything new into your work, so you don't need to feel that you need to find some new and unique point to say about this character.
Therefore, your conclusion in a sense will be written for you already if you have completed your essay. You just need to maybe find a quote or something that you can start your conclusion off with that somehow nails what it is you want to say about Gradgrind's character. One of my favourite quotes in relation to his character, and of course one that relates to the massive change that he undergoes during the course of the book, is the way in which what he experiences in his later life has the impact of "making his facts and figures subservient to Faith, Hope and Charity." This neatly summarises the development of his character and would be a good quote to include in your conclusion.
You are amazing.