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Using trace net on gerber files to build up the netlist. Would be a shame if I had to do the tracing all over next time I open the pcb. I cannot find any information in the .pcb file related to traces.
you can export the traced nets to a layout and save this layout as GDS.
Another option is to use "Trace All Nets" from 0.26 (Caution: still somewhat experimental). This will produce a "netlist database" you can save and load. This database contains all the information for all nets.