The Bevel tool allows you to create chamfered or rounded corners on Grease Pencil strokes. A bevel is an effect that smooths out edges and corners. In design, bevels are useful for creating friendly-looking designs by softening sharp corners. To use the Bevel tool, make sure you are in the Grease Pencil object’s ‘Edit’ mode. Go to Point > Bevel.
Immediately after running the Bevel command, open the Redo panel at the bottom left side of the Viewport. In it, you’ll see three options: Radius, Threshold, and Steps.

Radius
Radius controls the radius of the rounded corners on all selected Grease Pencil Stroke Points. It’s upper limit is when the radius intersects the rounded corner of another Point in the Grease Pencil Stroke.
Threshold
Threshold is the minimum distance Grease Pencil Stroke Points can be before they are merged with other stroke points. This helps remove redundant points created by the bevel.
Steps
Steps are the number of divisions made to the bevel for smoothness. The higher the step count, the smoother your bevel will be.