OK, here are the dimensions of the brass peg in the 5-kg counterweight that came with my GEM45:
Length 34 mm
First 8mm of it are 8mm in diameter
Rest is 6mm in diameter
There is also a spring somewhere in the hole that makes the brass peg retract when the knob is loosened. However, other iOptron counterweights, including the 2.5-kg one that I bought for the same mount, lack the spring. I may try to add it. Off to the workshop now
For a substitute, you might try a 6mm bolt, 35mm long, whose head is less than 8mm in diameter or can be filed down.