Common PCB Gerber File Extensions and Their Meanings Gerber files typically consist of multiple layers, each represented by a different file extension. Extension Meaning Purpose .GTL Gerber Top Layer Top copper layer .GBL Gerber Bottom Layer Bottom copper layer .GTS Gerber Top Solder Mask Top solder mask openings .GBS Gerber Bottom Solder Mask Bottom solder mask openings .GTO Gerber Top Overlay To...