Came here with a group. Among us we had a garlic allergy, a nut allergy and celiac disease. We got an experienced waiter who took good care of us. I ordered simply (fish + veg) to stay safe. Our waiter caught that the kitchen had made a mistake with the garlic allergy and had the dish remade before bringing it to the table. Everyone got fed, no one got sick.
I had no issues, but I'm not sure how confident I am about their competence with regards to GF. Everything is made from scratch, so that's a plus. However, there was an item with wheatberries on the menu marked as GF. I told the server that was definitely not GF. After speaking with the chef, she returned to say it was an error in the menu and that they would fix it immediately. Not sure how often that happens, but it made me quite wary. Unless you're aware of restaurant pitfalls and good at advocating for yourself, I'd stay away from Heirloom. That said, the meal I ordered, the curry quinoa bowl, was delicious and I had no adverse reaction.
To be honest I had no idea this restaurant even existed until I attended an event in the adjacent gallery. After noticing that the menu clearly marked the GF items though, I decided to give it a try. I had the salmon, which was very filling, although not quite great. Things took a turn for the better though with dessert - they have a GF pear cobbler [contains dairy, fyi] that I was absolutely incredible (and also huge). I'd try it again.