If you migrate your Jira instance from Server to Data Center by preserving the existing Jira data, there are no special steps to follow in order to migrate Great Gadgets app. After applying the new Data Center specific license key, the app will continue to work as before.
If you migrate to Data Center without preserving the existing Jira data, this will be like a new app installation. You will have to create new Jira dashboards and configure the app gadgets from scratch.