A wartime G3 would carry stronger antiaircraft defenses

Would the G3s' original six 4.7-inch antiaircraft guns have made them easy targets for the Luftwaffe?

No. The Luftwaffe did not exist when the ships were designed, and surviving G3s would likely have received stronger dual-purpose guns and carrier support before wartime service.

The original six 4.7-inch antiaircraft guns are not a fair measure of how a surviving G3 would face the Luftwaffe. Germany had no air force when the ships were designed, and the G3s would probably have been modernized before the Second World War. A likely refit would replace the older weapons with a larger battery of 4.5- or 5.25-inch dual-purpose guns, comparable to refits of older British capital ships. After the Norwegian Campaign, such ships would also be expected to operate with carrier support, making an attacker find the capital ship within a task force rather than alone.