The first part of the job is to smooth all the surfaces that might be rubbed if the shim wasn't there. Then shim.
The GPMC was advertised as having shims on hammer and trigger. Mine only had them on one, I think the hammer. I figured out how to make my own shims. (It's not totally straightforward to drill a round hole in 0.002 inch thick brass; think it through and use a few decades of experience making things.) Now I put them in anything where I think they'll help. Taking the lateral rattle out of a trigger makes a surprising difference in how it feels, even apart from the friction reduction. I went and bought a shim making punch tool on fleabay to make better ones faster, but haven't gone through any new pistols since buying it.
Per Iowegan's advice, I try to leave ~0.002" clearance. Measure the clearance, subtract 0.002", divide by two and round down. K&S sells a kit with a reasonable amount of stock in several thicknesses.
#258 .001,.002,.003 & .005 Shim
K & S PRECISION METALS 258 BRS
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Not another dime.