Don't know if it is still available, but in the 1970's I used a 'squashable material' - Plastigage - to check deck clearance. It came in at least two ranges, or at least I still only have two...:0.002-0.006" and 0.004-0.009". It was a Perfect Circle product, wih an address as Hagerstown, MD (no further details.)
As I said, I don't know if it is still around, or if so, is available in a suitable range for your G21/35. Simple enough to use: lay a short length of it on the piston where you want to know the deck clearance, turn it through TDC and remove it. Compare its squashed width with bars printed on the sleeve it ships in.
The thin round-section strips are color coded to reflect their range.
By the way, it may be quicker to cut head shims out of light aluminum sheet, like that used on pull-tab coffee can lids, disposable oven roasting pans, and such. These are a soft, easily cut aluminum.
A quick and dirty example: mark the bore diameter on the stock and use a 1/4" paper punch to remove most of the material inside the line. A Dremel red point can get right down to the line; debur the ground edge. Center the hole on the sleeve and mark your OD to fit. Tinsnips can get you close enough to the line.
Make sure the 'shim' clears the other pieces for assembly. Stacking as many as needed to make up your desired clearance is as good as turning a very thin piece, and both quicker and less messy.
You can do about the same with hardware store 0.020" roofers' 'flashing' aluminum, but might wear out a few paper punches... and that's several times the thicknesss of the can lids/roasting pan material. You might need a thickness that doesn't measure in units of 0.020"...