Lights

This appears as though the spotlight is not actually pointed to the plane of the bubble - remember this is a 2D object, not a sphere. Try angling the spotlight towards the plane of the bubble (I am assuming this is X-Z, so if you rotate the lamp along its X axis?) I prefer to use point lamps, as these will cast a radial glow that is not directional. Proximity to the Grease Pencil object, as well as intensity is what will give you the glow effect.

Hi Marcel,

Maybe try increasing the Light Power to something greater, like 3000 or so. In the case of a Sun Light, try 7, or even 10.

Just a dumb troubleshooting tip to check: your grease pencil object - do the layers you wish to light have "Lights" ticked?

I'm also assuming you've created this bubble in the default orientation (X-Z) so if you're using a point lamp, try moving it along the Y axis (You can do this my hitting G-Key to grab, then Y to constrain movement to only the Y axis). If it is behind the GP Object, it won't show anything. If it is in front, usually when it is really close, this should show the light effect. Once you have a position, tinker with the intensity and further position until you get a result you like.