Gas struts should be mounted with the 'skinny bit' (shaft) pointing down (even when closed). This keeps the
oil on the seal which prevents the gas leaking out. Regassing is done by putting the whole strut into a pressure chamber { sealed pipe} and pressurising that to the required value...the gas then forces its way past the seals in the strut and the job is done.
I forgot to add...if they are really high pressure. High temps might cause the strut to vent past the seals.