The night before, make sure your ice cream maker bowl is in the freezer.
In a small bowl, whisk gelatin, matcha (see notes below on how much to add), and sugar.
Add the 1/2 cup cold water and mix well. Set aside.
In a large pot, combine the whole milk and heavy cream. Heat over medium high heat, stirring occasionally, until the mixture is boiling.
Turn off the heat and add the matcha gelatin mixture. Stir until everything is dissolved. It's okay if there are a few tiny clumps of matcha. Strain the warm mixture into a large bowl to catch any clumps of gelatin or matcha. Put mixture, covered, in the fridge until cool to the touch, at least an hour. This step can also be done overnight to churn the ice cream the next day.
Prep your ice cream churner, removing your bowl from the freezer. Pour the cooled matcha cream mixture into the machine and churn for 20 minutes or until your ice cream reaches soft serve consistency.
You can eat immediately at this consistency, or put in a container and let harden in the freezer. Before serving, let ice cream thaw for 10 minutes on the countertop.