Layer-function extensions such as .gtl, .gbl, .gts, and .gto are useful hints, but they are conventions rather than part of the Gerber specification — the Gerber spec itself only defines .gbr (or .GBR) as the standard file extension. The Gerber file content, not its extension, defines the image. The layer-function extensions are a naming convention used by EDA tools, manufacturers, and CAM operators to recognize layers quickly.
This distinction prevents a lot of confusion. A file named .gbr can be a perfectly valid top copper file. A board outline might use .gko, .gm1, .gml, or a descriptive name ending in .gbr. Some KiCad Gerber X2 exports use .gbr for multiple layers and rely on embedded attributes plus filenames to describe function.
If you need the broader beginner explanation, read what a Gerber file is. If you only need to inspect a received package, the Gerber opening guide covers viewer workflows.
Quick answer
Common Gerber extensions are conventions, not mandatory standards..gtl usually means top copper, .gbl bottom copper, .gts top solder mask, and .gto top silkscreen. The safest confirmation is to open the package and inspect the rendered layer content, filename, and any Gerber X2 attributes.
Why extensions are only conventions
The Gerber specification describes file syntax and image data. It does not require every top copper layer to end in .gtl, and it does not make a file top copper just because someone renamed it with a top-copper-looking extension. Many of the familiar extensions were popularized by Protel and Altium-style naming, then adopted widely because they are convenient.
In real packages, layer identity often comes from a combination of clues: the extension, the base filename, Gerber X2 file attributes, the folder name, a readme, and the rendered image. A robust review process checks the content instead of trusting one clue. This is especially important for board outlines, mechanical layers, drill drawings, and inner layers, where naming varies more between tools.
Common Gerber and drill extensions
| Extension | Common meaning | What to verify |
|---|---|---|
| .GTL | Top copper | Top-side traces, pads, pours, and copper features. |
| .GBL | Bottom copper | Bottom-side traces, pads, pours, and copper features. |
| .G1, .G2, .G3... | Inner copper layers | Layer numbering and stack order from the EDA export. |
| .GTS | Top solder mask | Mask openings where top copper should be exposed. |
| .GBS | Bottom solder mask | Mask openings where bottom copper should be exposed. |
| .GTO | Top silkscreen or overlay | Top legend text, outlines, polarity marks, and labels. |
| .GBO | Bottom silkscreen or overlay | Bottom legend content, usually mirrored for bottom view. |
| .GTP | Top paste | Stencil openings for top-side surface-mount pads. |
| .GBP | Bottom paste | Stencil openings for bottom-side surface-mount pads. |
| .GML, .GKO, .GM1 | Board outline or mechanical | Profile, slots, cutouts, dimensions, or mechanical references. |
| .GD1 | Drill drawing | A drawing or map, not necessarily machine drill data. |
| .GBR | Generic Gerber | Any layer; use filename, X2 attributes, and rendered content. |
| .DRL, .TXT, .XLN | Excellon drill, not Gerber | Hole coordinates, tool diameters, PTH or NPTH intent. |
Inner layers and numbering
Inner copper layer naming is one of the easiest places to make a wrong assumption. Some exports use .G1, .G2, and later numbers for internal copper. Others use names such asIn1_Cu.gbr and In2_Cu.gbr. The number does not always tell you the physical stack order without context, especially when a tool or fabricator uses a different convention.
For multilayer boards, include a stackup note or layer map. The map should identify top copper, each inner layer in order, bottom copper, solder mask, silkscreen, paste, outline, and drill files. That small text file removes ambiguity even when the Gerber files themselves are technically valid.
Board outlines vary by tool
Board profiles are often named as outline, edge cuts, keepout, mechanical, route, mill, or dimension layers. Common extensions include.GKO, .GM1, and .GML, but none of those names is universal. Some packages include multiple mechanical layers, and not every mechanical drawing is the routed board edge.
When checking a package, confirm that the outline is closed, matches the expected board size, and aligns with copper and drill data. If the package contains both a dimension drawing and a fabrication route layer, make sure the manufacturer can tell which file is the actual board profile.
How to confirm what a file really is
Open the complete ZIP in the Gerber viewer and inspect each layer visually. Copper layers show traces, pads, pours, and clearances. Solder mask layers show openings around exposed copper. Silkscreen shows printed legend. Paste layers show stencil apertures, usually only for surface-mount pads. Drill files show holes and tool sizes rather than Gerber image artwork.
Do not rename files blindly to fit a list from the internet. If the contents are correct but the names are ambiguous, add a readme or layer map. If the contents are wrong, regenerate the fabrication output from the PCB tool with the correct layer selections.
FAQ
- Why do my Gerber files all use .gbr?
- Some tools use a generic .gbr extension for every Gerber layer, especially when Gerber X2 attributes or descriptive filenames identify the layer function.
- Is a .drl file a Gerber file?
- No. A .drl file is normally an Excellon drill file containing hole coordinates and drill tool information, not a Gerber image layer.
- What if my extensions differ from the common list?
- That can be normal. Layer-function extensions are conventions rather than part of the Gerber specification (which only defines .gbr/.GBR as the standard extension). Open the files in a viewer and check the rendered content and any X2 attributes or layer map.
- Do PCB manufacturers require specific Gerber filenames?
- Many manufacturers accept common naming conventions, but requirements vary. Follow the manufacturer instructions and include a layer map or readme when names are not obvious.