the way the CI runners on tangled work, you could just plug in your own bespoke runner as long as it fits the interface. we implement two such "engines": nixery and microvm. you can plug an engine like tack[0], which can act like a bridge interface to other CI systems. there is also loom[1], which is a kubernetes based engine.
reply[0]: https://tangled.org/mitchellh.com/tack
[1]: tangled.org/evan.jarrett.net/loom
The problem is that interface isn't enough when in Gitlab the CI natively integrates with other systems, like test reports displaying results in merge requests. This would certainly enable hybrid pipelines through.
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