From May 11, 2020 to May 18, 2020, it was possible to find G-Max Pikachu in Max Raid Dens. Another way to get G-Max Pikachu is to use Max Soup on it. If you have the Isle of Armor DLC, there is an item called Max Soup that can be applied to Pikachu (and other Pokemon with G-Max forms) that gives it it's G-Max form.
Max Soup and the Let's Go! Pikachu prize are currently the only ways to obtain G-Max Pikachu.