annotated gds format
You would be looking for an "extract" or layout based
netlister. You might have to place some sort of pin
polygon or text, if you want a non-gibberish set of
net names - this all depends on how the tool recognizes
nets. There will be some body of recognition rules you
will have to write if you don't use the foundry PDK and
supported tool set. Foundry PCells ought to have some
data features that make the extract tool's life easier,
but these layers may or may not stream out with
the mask geometry data (they tend to be non-print
layers).
I know the LASI tools will do what you want, after
import of the GDS data. Electric requires some "arc"
constructs to be used which are destroyed by the
stream-out process and LVS won't work on non-
native layouts there. There may be other layout
oriented free / open source tools (like the Dolphin
suite, or Paris/MGEN) that will do it. I have not run
across any standalone tools but that means nothing.
If annotated GDS is in use, it's not in common use
(not an option in Cadence current release stream
I/O). You'd want to know if whoever made the PDK
has put in the right support for net and pin names,
in conjunction with how your eventual selection of
extract tool would recognize same.
I suggest as a start that you Google "layout netlist
SPICE" and similar keywords, if these tools I list
aren't good for your job.