Because the piston and piston rod are pressed in to the cylinder, volume reduces and pressure increases. This will cause pushing force to increase. In conventional gas-type springs, this increase is normally around 30% at full compression.
The pushing spring movement is slow and controlled. It's reliant on the gas flow between the piston sides being able to move through channels within the piston throughout the stroke. Conventional gas springs use 'hydraulic damping', which involves a small amount of oil slowing the pace from the stroke immediately before the spring reaches full extension. This gives the movement a braking character by the end position so long as the piston rod is in the downward direction.
Force tolerances when charging with gas and other factors signify there might be variations within the force exerted by gas springs with the same nominal value.
The nominal values apply at 20° C, which is temperature where gas charging is carried out. Remember that in the event the ambient temperature rises or falls, the force with the spring rises or falls based on pressure changes in the cylinder. Generally of thumb, gas spring force increases by approximately 3.5% per 10° C temperature increase and reduces accordingly once the temperature falls.