[hgomersall]

We've adopted this policy internally. It largely requires using the proper git workflow, rather than (in our case) gitlab's GUI flow, though we make an exception for a simple merge (I think it's verifiable that no code is added).

The check is that commits at the point of merge have a valid signature. Historical commits are part of the history and as such cannot be changed without an additional commit (with a valid signature). Previous unsigned commits are deemed trusted at the point you begin signing and checking.

Squash merge breaks stuff and shouldn't be used. To complete things, the restricted set of operations exposed through the GUI should sign using gitlab's or github's key (or some accepted bot key we've set), with the check happening on the input commits, but AFAICT that's not supported yet.