That's not how healers work. You have a wide range of oGCDs that are instant cast and can be weaved while doing dps. Only when you've exhausted all of them and the tank is in actual danger is it worth stopping your dps to focus GCD heals.
That last scenario does happen, especially in some leveling dungeons with undergeared tanks, but it's a last resort scenario after you've exhausted your main options.
Your priority is backwards. It should be "I main dps (with oGCD heals) until it seems the tank isn't good, then I'll heal some". If you aren't going to at least try to contribute everything you have, don't sign up for team based content.