I was asked to help fab a demo board that is posted on the Analog Devices website. They did the layout in PADS (apparently) and it generated files with xxx.pho names. JLC couldn’t read these files. After some effort looking at the raw files, it became clear they are real Gerber files but not with names that JLC would recognize. The KiCad Gerber Viewer did display them without any complaints.
I’m not sure where to post this so folks can find it, but here is the translation table.
TM.pho abc.GBS TM = top solder mask
BM.pho abc.GBS BM = bottom solder mask
L1.pho abc.GTL Top Layer copper
L2.pho abc.G2L L2 coppper. L3=G3L if there is a 3rd layer
L4.pho abc.GBL bottom copper layer
TS.pho abc.GTO Top silkscreen
.pho refers to a “plot” file. There must be some history to the .Gxx extensions too. Using this as a guide, you can likely decode other filename to extension conversions.
Hope this helps someone someday.