Ooh sweet innocent anonymous! Do you have any idea how many celebrities have been and still are secretly dating? Best exemple I have in mind now: Johnny Galecki and Kaley Cuoco. And they’re A-listers, which is not the case with David and Gillian.
Of course it’s totally doable. You just have to not being wandering around in the streets of Los Angeles and make sure your girlfriend doesn’t call the paparazzi or tell all her friends on Instagram where they will be at the day after.
I’m 100% sure that vacations and date nights happened between Gillian and David for years. They just know how to be cautious, which is what celebrities do when they don’t want people to know what’s happening in their private life. And in case they’re not cautious enough, which was the case in 2012, with all these sightings and everything, they have lawyers to take care of that in the most possible discreet way.

