The quick way would be to disconnect your Z motor, then use a spring scale (fish scale) to weigh the Z slide assembly. Look at the force required to pull it up. That will get you in the ballpark.
If you don't have a lot of movement in your Z axis, a coil spring might work. But the force of a coil spring is not linear, so having a really long spring relative to the travel would be in order. Not the best system
Gas struts are generally hydraulically damped, so do not work well where you have a lot of fast movements required.
The best way to do it is to use an adjustable air spring. This consists of an air cylinder, a small air tank, and a regulator. 5 gal portable air tanks are available at your local Harbor Freight or auto parts store. Air cylinders are available from Automation Direct, McMaster, Grainger, etc. as are regulators. You just size the air cylinder so you can run it at 30 or 40 PSI to get the amount of lift needed. Supply regulated air to the tank, and Tee the air cylinder off the tank side of the regulator. The tank is to maintain a constant pressure on the air cylinder, just the regulator alone is not adequate, it won't react fast enough. I'm using this system on my machine and it works great!