4-Layer PCB Plane Layers: Positive or Negative Gerber?

4-layer PCBs have two internal “plane” layers. The industry standard for the ground plane and power plane (layers 2 and 3) is negative (inverted: filled where there is no copper).

What format does Seeed Studio use for Fusion PCB? Should the .GL2 and .GL3 be positive or negative (inverted)?