Your shop gave you bad advice. Garmin says that the frequencies are sufficiently different, even counting various harmonics, that interference wouldnt be an inherent problem. They do, though, recommend a minimum of two feet between a GPS antenna and any strong transmitter. There are few, if any, airframes where it would be impossible to get at least that. If your tech, however, has seen that problem, its likely due to other factors, like poor antenna grounding, poor connections, bad wiring, etc., but not antenna proximity unless they were right next to each other.