It can be tricky to get an excellent first layer with PETG. It is prone to warping, and to avoid any print failures; you need to ensure that the first layer adheres to the bed flawlessly. A heated bed will help to minimize the temperature gradients and help PETG stick to the bed. If you’re using a glass bed surface, you can apply a thin layer of PVA glue stick or hairspray to achieve optimum first layer adhesion. Alternatively, you can also stick the Blue painters’ tape on the glass print bed.