Open-air art exhibits are great things! But they do leave the question of what to do if one needs the bathroom, open. Here is a map of some places in walking distance to Parc Gallery. They are cafés and restaurants, including one particularly child-friendly café, called (Parent)These, on rue Notre-Dame. Everything is within a 5 to 12 minute walk from the park.
Many thanks to Kathleen for pointing out that, for visitors coming to Parc Gallery via the canal path, there is a public facility at the Parks Canada building at the St Gabriel locks. Much appreciated information!