My goal is to create an animation of a boids particle system. Each boid should be an instance of a 2d grease pencil animation. The problem is that the grease pencil animation stops before the boid animation is done. For regular animations, I know it is possible to make them loop within a longer animation by applying a cycles modifier in the graph editor, but I can't get my grease pencil animation to show up there.
I figured it out thanks to wilks' comment above.
It can be done by adding a time offset modifier to the grease pencil object,
custom range and setting
end frame to the length of the grease pencil animation.
Only problem now is that all the boids loop in unnatural unison, but I guess I'll open another question for that.