My main complaint that I use the Slippery Elm for was gastritis. I have noticed an increase in acid reflux / nausea while on the meds. My doc gave me metoclopramide for the nausea, which is for speeding up the passage of food, however that comes up orange with led/sof on the drug interaction site. I never took it anyway, just not into loading my liver up with even more drugs if I can help it, and the nausea was doable.
The thing I find most helpful for acid reflux is organic apple cider vinegar, unpasteurized, with the 'mother' in it. Bragg make one, also Biona. I used that before tx and sometimes on tx but not close to the time of taking my meds. I am not sure it is ok with the DAAs but probably ok with led. Dac seems to be more sensitive to various foods.
I use it by taking about 10ml in 50 ml of water, 20-30 mins before a meal. It seems to tune the digestion up no matter if you have too much or too little acid. You probably need to do it for a week before meals to see the full benefit, but even once can make a difference.
Sorry, I don't know about gaviscon with the DAAs.