[MadMax]

Use a file and file two crossed lines with the corner of the file in an "X" so your drillbit doesn't wander around the round surface of the cylinder. Start with a small hole around 1/16" to pilot your final hole diameter (I'm guessing around 1/8"). Do not increase hole dia in very small increments.

Brass is soft so drillbits can grab and pull the part aggressively. Drilling in small increments invites this. I suggest stuffing a hunk of wood or at least stuff a wad of paper folded over a dozen times (12 layers or a bunch more) in case this happens so the drillbit does not mar the other side. Use a drill press vise to secure the cylinder to prevent the part from helixing upwards.

Contrary to intuition, it's better to spin the chuck fast when drilling brass with a sharp bit. A fast bit (not super dremel fast, but high speed on a drill press) will cut thinner chips and leave a thin final chip when it breaks through. If the last chip is thin, it has less strength to grab the drillbit and will more likely shear instead of riding the helix flute up.

Use sharp bits to reduce the upset bur left on the inside. A dull bit will mash a heavier burr into your hole.

A burr left on the inside of the cyl will nick your piston oring. I suggest running the tip of an xacto (not an Olfa box cutter which has a completely different tip angle) around the inside edge of the hole to clear away the sharp burr. Scrub the hole from the inside with extra fine steel wool. You may lightly scratch the inside of the cyl, but you will probably not damage the oring seal with the slight finishing. Wash out all loose burrs and steel wool bits scrupulously before reasembling your Gspec.

If you want, practice on a hunk of 3/4" copper pipe. It's softer, but if you've never drilled a metal tube before, you don't have to risk your Gspec cyl. There is also brass tubing at your local hobby store to goof around with. I think you should be able to find up to 3/4" there.