You don’t really have z-fighting in grease pencil unless you are using multiple GP objects. I would separate the fills from the lines in a separate layer. So since I don’t know your setup I’m going to give some general guidance. If they are multiple object then just move the objects slightly in the 3D space. For example: select the object you want to move and press G->The axis you want to move->type something like 0.002 or -0.002. If they are separate layers then reorder the layer. Goto the layers panel and select layer and press up or down button on the right. This panel can be found in the top of the header or the properties editor->Data/Grease Pencil tab. If they are in the same layer then you can select the GP and press tab to enter edit mode. Select the fill or line(Select a control point/vertex and press L or switch to stroke selection by pressing 2 on top row.) Then you can move the order by using Ctrl+arrow up/down or you can move to a new/different layer by pressing M.
