I was a single dad with sole custody of a child by court order for many years now. Recently I got married again, is my new wife automatically her guardian in the event of my death.

Unless she has adopted the child, then no.

Usually custody reverts to the other parent in the event of the death of the custodial parent.

No, even though she married you, she has no legal status to the child what so ever. She is not the guardian now, or then.

The child's other parent would often have first chance, and then other family members could get guardianship, aunts, grandparents, and so on.

The only way to give her any legal status would be for her to adopt the child, with permission of the other parent