Read up on banding and how to avoid it.
Thank you very much! As a non-native it's not always easy to find correct terms for troubleshooting.
You have two choices. First increase the project bit depth to 32 bit. That will remove the banding but the banding may still be visible on your display because it may not be running at a high enough bit depth to prevent banding.
If you are planning to render to an 8 bit codec for delivery then you need to be very careful with gradients like this. The best way to hide the 8bit banding is to add a little noise. You don't need much. Even a Fractal noise overlay would help without making the 'glow' look noisy.
Thank you for the advices.