Atlassian does not support migrating dashboards with custom gadgets from Jira Server & Data Center to Jira Cloud. This feature is a top priority for our development team and we are actively working with Atlassian on this functionality.
If you want to migrate your Cloud instance manually, you can use the built-in Import / Export Configuration feature.
Do not select the Custom Charts option when using Jira Cloud Migration Assistant (JCMA) for your migration. Selecting it will cause the migration to fail.