git-bug, the one mentioned in the article here, has some documentation on its README of how well its importer/exporter tools support Github, Gitlab, Jira, and Launchpad: https://github.com/MichaelMure/git-bug
Most of the other such tools I've seen barely have the resources to import/export a single such API. git-issue only has Github import it looks like. https://github.com/dspinellis/git-issue
There's perceval which is designed to be a generic archival tool and supports lots of APIs, but only dumps them to source-specific formats and would still need a lot of work if you tried to use issues from different APIs together: https://github.com/chaoss/grimoirelab-perceval
Most of the other such tools I've seen barely have the resources to import/export a single such API. git-issue only has Github import it looks like. https://github.com/dspinellis/git-issue
There's perceval which is designed to be a generic archival tool and supports lots of APIs, but only dumps them to source-specific formats and would still need a lot of work if you tried to use issues from different APIs together: https://github.com/chaoss/grimoirelab-perceval