Is not possible to import issues directly from GitLab to JIRA. But if you can export issues from GitLab to a CSV or JSON file, then you can use this file to import the issues to JIRA, as per https://confluence.atlassian.com/display/JIRA/Importing+Data+from+CSV
Unfortunately the import wizard does all sorts of things you don't expect, like space separated labels, and auto-creating users, and getting the author wrong, and all sorts of stuff.
At the end of the day, the conversion almost never works and is extremely slow and painful to to undo becase Jira has no bulk user editing.
A custom program needs to be written for gitlab... don't waste your time trying to get the import to work, unless you heavily massage the data in excel first.
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