To carry Gretchen around you need to press when she is near Heinrich and he will pick her up. While carrying her, both of them will heal at an accelerated rate and you don't need to worry about Gretchen lagging behind Heinrich and taking damage. To unlock this achievement you need to carry her for 2 hours. You don't need to move around at all and the achievement does not need to be done all at once. If you're going for all difficulties then this should come naturally during play. If you're really desperate to get the 20 then you can pick Gretchen up in a safe area and go do something else for a couple of hours. If you want to do it this way you'll need a wired controller or play and charge kit so the controller doesn't turn itself off.