Altium Layer and Gerber File Extension Key

.txt - Drill file
.gbl - botom layer
.gbs - bottom solder mask
.gbo - bottom overlay (silk screen)
.gbp - bottom paste
.gko - keep out
.gm1 - mechanical 1 (board outline)
.gm2 - mechanical 2
.g1 - inner routing layer 1
.g2 - inner routing layer 2
.gp1 - inner plane 1 Negative
.gp2 - inner plane 2 Negative
.gpb - bottom pad master
.gpt - top pad master
.gtp - top paste
.gtl - top layer
.gts - top solder mask
.gto - top overlay (silk screen)

Other Miscellaneous Files:

.rul - rule report file
.ldp - layer report file.
.apr - aperture report file
.drl - unknown binary drill file
.drr - drill report