hi,friend:
If you truly study Gc-place,I think you must know below basic information:
one: you know to judge the format of gerber(RS-274-D OR RS-274-X)
two:you can understand the theory of photo plot;
three:you need understand the "trace,pads,parts";
Four:you must know the "Process layer and physical layer"
Five: you start to learn the converted way:
For example Trace--->Pads
PADS--->PARTS
Identify Parts
Good luck for you!