.GBL -> Bottom Layer (Copper)
.GBO -> Bottom Outlines (Silkscreen)
.GBS -> Bottom Stop (Solder mask)

.GKO -> Milling

.GTL -> Top Layer (Copper)
.GTO -> Top Outlines (Silkscreen)
.GTS -> Top Stop (Solder mask)

.XLN -> Drilling (Excellon)