More details please. Have you dropped the trigger group and removed the cylinder and crane assembly?
The cylinder and crane assembly on the GP100 is not the same as the SP101 and the cylinder will not slide off the crane. There is disassembly of the crane requiring two small 1/16" punches and then a special bit to unscrew the threaded cylinder retainer, which is threaded counter-clockwise.
Why do you need to remove the cylinder on a new gun? Are you having an issue? It is not necessary to disassemble a GP100 to this level for routine cleaning.
Here's a picture that may help. The spring loaded plunger pushes in (right punch in photo) then the Pivot pin is pushed out with the punch on the left. You don't have to remove the left hand thread retainer screw unless you want to remove the ejector (star wheel). I agree with Waveform …. there's really no need to remove the cylinder unless there is a problem.