1. Gerber files needed for 1-layer PCB boards:

Bottom/Top layer: pcbname.GBL/GTL

Solder Stop Mask Bottom/top: pcbname.GBS/GTS

Silk Bottom/Top: pcbname.GBO/GTO

NC Drill: pcbname.TXT


2. Gerber files needed for 2-layer & 4-layer PCB boards:

Top layer: pcbname.GTL

Inner Layer 1: pcbname.GL1 (for 4-layer board)

Inner Layer 2: pcbname.GL2 (for 4-layer board)

Bottom layer: pcbname.GBL

Solder Stop Mask top: pcbname.GTS

Solder Stop Mask Bottom: pcbname.GBS

Silk Top: pcbname.GTO

Silk Bottom: pcbname.GBO

NC Drill: pcbname.TXT

Outline layer: pcbname.GKO or pcbname.DO


Note:

  •  All the gerber files must be in RS-274x format, except the drill file should be in Excellon format
  •  If there is no pcbname.GKO or DO file, please make sure the PCB outline or shape shall at least be specified in other layer, silkscreen or soldermask layer etc. 
  •  If soldermask layers are not required, please clarify surface finish for both sides when you submit your gerber files. 



How to generate Gerber files from KiCad or Eagle


http://blog.iteadstudio.com/how-to-generate-gerber-files-from-kicad/

http://blog.iteadstudio.com/how-to-export-gerber-files-from-eagle/