nit: Have we considered naming like:

• GcRuleUnionBuilder
• GcRuleIntersectionBuilder

I think it may be clearer for users that these are specifically for GcRules. I guess that in the context it will generally be:

GcRuleBuilder.union().addRule(...)

or

UnionRuleBuilder ruleBuilder = GcRuleBuilder.union();
ruleBuilder.add(...)

which is also pretty clear.