Break it into subjects and brainstorm how you can relate information to that subject.
Math - distances travelled, amount of money made, price per weight/size
Social studies - timeline, where people travelled from, what happened to the families they left
Science - how does gold form,
LA - make brochures to encourage people to move, write letters to family back home from the perspective of someone who followed the gold rush
If all else fails, find a lot of good trade books and develop ideas from there.
Children of the Gold Rush by Claire Rudolf Murphy and Jane G. Haigh
The Bite of the Gold Bug: A Story of the Alaskan Gold Rush by Barthe DeClements and Dan Andreasen