Havoc OS on Fairphone 3

Can I use this on my Fairphone 3?
Sorry if I chose the wrong Topic btw.

it’s a version of a custom ROM for a Samsung phone, which the Fairphone 3 is not.

Aren’t the GSI builds compatible with any Phone?

With any Project Treble compatible phone, yes, supposedly so.
Mind that GSIs are experimental in nature. They might work well on a device, or not.

Do they work in this case?

I don’t know, hopefully somebody else can provide an answer.
I’m not too convinced of the concept anyway. Why use experimental stuff like GSIs if well-maintained “real” OSes are available? See oslist.

GSIs have their use, using them as a daily driver is an option some choose, and they might work well, but it’s simply not their intended use:

"App developers can install and run the latest Android GSIs to perform app testing on a variety of existing Android devices and using GSIs from different Android OS release stages, including Developer Preview and Beta builds. Adding GSIs to your verification and testing processes can provide you with some extra benefits:

Broader test coverage on a greater set of real devices
More time to fix app compatibility issues
More opportunities to fix compatibility issues in Android that are reported by app developers

The GSI project is open source and helps improve the Android ecosystem by providing more ways to improve app and OS quality before each release of Android."

“Android Generic System Images (GSIs) are for app developers to perform app validation and for development purposes.”

“GSIs are used for running VTS and CTS-on-GSI tests. The system image of an Android device is replaced with a GSI then tested with the Vendor Test Suite (VTS) and the Compatibility Test Suite (CTS) to ensure that the device implements vendor interfaces correctly with the latest version of Android.”