Boil potatoes, garlic, and bay leaf until the potatoes are tender.
Remove bay leaf, drain, and return potatoes and garlic to the pan, shaking over medium heat until residual water evaporates and the potatoes look dry.
Off heat, puree the potatoes with a masher or ricer, taking care to work out all the lumps.
Place pan over low heat and add Gruyere, buttermilk, and butter. Mix until the cheese and butter melt, increasing the heat slightly if needed to help the cheese incorporate smoothly into the mixture. Season with salt. If desired, beat potatoes briefly with a hand mixer or in the bowl of a standing electric mixer fitted with the paddle attachment to make sure they're extra smooth.