Well, in my opinion IIT Dhanbad is far better than IIT Gandhinagar for mechanical engineering...
Well, IIT Dhanbad provides good placements as compared to that of IIT Gandhinagar.
IIT dhanbad provides more research opportunities and international exposure and Flexible Curriculum (studying mechanical engineering you can pursue courses/electives in Management, Cognitive Science, CSE, Electrical, Physics, Humanities and develop proficiency and widen your knowledge spectrum).
I hope this would help you. For further queries please feel free to write me down in the comment box.
IIT gandhinagar is better than IIT dhanbad for mechanical engineering course. IIT gandhinagar has best faculty for teaching, best lab equipment for praticals, best facilities, best placements with high paid salaries. So, IIT gandhinagar is far better than IIT dhanbad. Hope it helps for you.