If you realy want to write your own g-code interperter then you can look at EMC, www.linuxcnc.org, wich is opensource.
I believe it was based on code released by the NIST (some kind of US govermental org.).
As far as I know there is no opensource code around for a g-code interpreter in an embedded aplication.
You can find some free stuff on www.cncdudez.com (from another member of the zone) but no source.
I'm still learning to code for embedded systems but like Carel I prefer Avr over Pic, however most (cnc)hobiests seem to use Pic.
Maybe it's better to get your machine running first with a DIY driver like the UHUservo and software like Mach3 or EMC and then build a g-code iterpreter.
Succes,
Jeroen
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Jeroen