Preamble
Gating is basically a way ensuring that master is always buildable even when cross repository dependencies are used. See upstream documentation about Zuul: http://docs.openstack.org/infra/zuul/gating.html#cross-project-testing

(Or, if you're more into having someone explain it to you with voice+slides, see: https://www.youtube.com/watch?v=sLD9LHc1QFM )

The current world
Right now we are working to integrate Differential and Gearman. (see T130949). This gives us Nodepool support for Differential change testing but it does not give us cross repository dependencies and gating.

Option 1: Differential support in Zuul???
There are rumors (@hashar can you link to any public documentation/plans from James E. Blair) that newer versions of Zuul will support non-Gerrit review systems as a 'trigger'. Without a timeline from upstream Zuul this is not something we can depend on.

Option 2: Gating support in Harbormaster???
Open question that @greg can't tell from a quick look at upstream documentation: does Harbormaster support cross-repo gating in a similar fashion as Zuul? Do you know, @mmodell?

Option 3: Push the 'gating' to before deploying???
Another option is not using gating and instead pushing back this functionality to A) watching to make sure Beta Cluster doesn't break and B) adding a pre-deploy review of a big cross-repo integration job that runs at some interval (hourly?).