Given how good Gabriels are at pouncing on stragglers and isolated units, and the obvious counter to them being to bunch up and turtle for a few turns while you whittle down their numbers, I want to pair them with stuff that punishes such turtling, or takes advantage of it. Artillery units and things that can serve liberal doses of Swarms are obvious choices, though high altitude bombers* can also be useful, as can ginormous linebreakers such as Banshees that are designed break up dense formations with the threat of powerful pointblank attacks.
*emphasis on HIGH, people. Dense formations are murder inn any aircraft dumb enough to fly low enough to be shot at conventionally.
